NASA Technical Reports Server (NTRS) 20170001939: High-Resolution IR Absorption Spectroscopy of Polycyclic Aromatic Hydrocarbons in the 3-micrometers Region: Role of Periphery

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by  NASA Technical Reports Server (NTRS)

High-resolution IR absorption spectroscopy of polycyclic aromatic
hydrocarbons in the 3-µm region: Role of periphery
Elena Maltseva
University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
and
Annemieke Petrignani1,2, Alessandra Candian, Cameron J. Mackie
Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
and
Xinchuan Huang3
SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043, USA
and
Timothy J. Lee
NASA Ames Research Center, Moffett Field, California 94035-1000, USA
and
Alexander G. G. M. Tielens
Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
and
Jos Oomens
Radboud University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands
and
Wybren Jan Buma
University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
[email protected]
ABSTRACT
1RadboudUniversity,Toernooiveld7,6525EDNijmegen,TheNetherlands
2UniversityofAmsterdam,SciencePark904,1098XHAmsterdam,TheNetherlands
3NASAAmesResearchCenter,MoffettField,California94035-1000,USA
1
In this work we report on high-resolution IR absorption studies that provide a detailed view
onhowtheperipheralstructureofirregularpolycyclicaromatichydrocarbons(PAHs)affectsthe
shape and position of their 3-µm absorption band. To this purpose we present mass-selected,
high-resolution absorption spectra of cold and isolated phenanthrene, pyrene, benz[a]antracene,
chrysene, triphenylene, and perylene molecules in the 2950-3150 cm−1 range. The experimental
spectra are compared with standard harmonic calculations, and anharmonic calculations using
a modified version of the SPECTRO program that incorporates a Fermi resonance treatment
utilizing intensity redistribution. We show that the 3-µm region is dominated by the effects of
anharmonicity,resultinginmanymorebandsthanwouldhavebeenexpectedinapurelyharmonic
approximation. Importantly,wefindthatanharmonicspectraascalculatedbySPECTROarein
goodagreementwiththeexperimentalspectra. Togetherwithpreviouslyreportedhigh-resolution
spectra of linear acenes, the present spectra provide us with an extensive dataset of spectra of
PAHs with a varying number of aromatic rings, with geometries that range from open to highly-
condensed structures, and featuring CH groups in all possible edge configurations. We discuss
the astrophysical implications of the comparison of these spectra on the interpretation of the
appearance of the aromatic infrared 3-µm band, and on features such as the two-component
emission character of this band and the 3-µm emission plateau.
Subjectheadings: astrochemistry—ISM:molecules—methods: laboratory—techniques: spectroscopic
— line: identification
1. Introduction 2000)). Althoughsuchcoolingconditionsallowfor
an increase in spectral resolution as compared to
Polycyclic Aromatic Hydrocarbons (PAHs) are
room-temperature experiments, they lead at the
a family of molecules consisting of carbon and
same time to matrix-induced effects that are not
hydrogen atoms combined into fused benzenoid
well understood and hard to predict. Gas-phase
rings. From a chemical and physical point of view
studiesaremuchpreferredbuthavesofarpredom-
they have properties that have led to exciting ap-
inantlybeenrestrictedtoIRabsorptionstudiesof
plicationsinnovelmaterials(Wanetal.2012;Sul-
hot (1000 K) vaporized PAHs (Joblin et al. 1995,
livan & Jones 2008), but at the same time also to
1994) or at best under room-temperature condi-
quite a cautious use on account of their health-
tions(e.g.,Piraliet al.(2009);Can´eet al.(1997)).
related impact (Kim et al. 2013; Boffetta et al.
Due to their low volatility, high-resolution studies
1997). For astrophysics they play a particularly
of low-temperature, isolated PAH molecules has
important role since PAHs have been proposed as
foralongtimeremainedoutofreachwithasano-
main candidates for carriers of the so-called aro-
tableexceptionthecavityringdownspectroscopy
matic infrared bands (AIBs), a series of infrared
(CRDS) studies of Huneycutt et al. (2004) on
emission features that are ubiquitously observed
small PAHs, although contaminations originating
acrossawidevarietyofinterstellarobjects. These
from isotopologues and other PAH species or im-
emissionfeaturesarethoughttobenonthermalin
purities remained a point of concern.
nature and arising from radiative cooling of iso-
Recently, we have applied IR-UV double res-
lated PAHs that have been excited by UV radia-
onance laser spectroscopic techniques on PAHs
tion (Sellgren 1984).
seeded in supersonic molecular beams. In com-
Since they offer such a powerful probe for car-
bination with mass-resolved ion detection these
bonevolutioninspace,thesebandshavebeensub-
techniques allow for recording of mass- and
ject to extensive experimental and theoretical re-
conformation-selected IR absorption spectra with
searchwiththeultimateaimbeingarigorousiden- resonance band widths down to 1 cm−1 (Malt-
tification of the molecular structure of the AIB
seva et al. 2015). Under such high-resolution con-
carriers. Significant progress has been made in
ditions, IR absorption spectra of PAHs in the
this respect with infrared (IR) studies on PAH
3-µm region turn out to display an unexpected
species deposited in a cold (10 K) rare-gas ma-
largenumberofstrongbands, andcertainlymany
trix (for example, Hudgins & Sandford (1998a,b,
2
more than expected on the basis of a simple har- mandola et al. 1989). However, to what extent
monic vibrational analysis. Such a conclusion is this explanation can account for the appearance
more pertinent as theoretical studies of IR spec- of the entire plateau is still far from clear.
tra of PAHs are typically performed at the Den- Our previous study aimed at recording spec-
sity Functional Theory (DFT) level, using the tra under the highest resolution conditions possi-
harmonic approximation for vibrational frequen- bleandapplyingtheappropriatetheoreticaltreat-
cies and double harmonic approximation for in- mentincludinganharmoniceffectsandresonances.
tensities, neglecting the effects of anharmonicity. For that reason, we focused on the spectra of
In our previous papers we demonstrated that a thelinearPAHsnaphthalene(C H ),anthracene
10 8
propertreatmentofanharmonicityandFermires- (C H ), and tetracene (C H ). As discussed
14 10 18 12
onancesindeedleadstopredictedspectrathatare above, astronomical spectra likely comprise the
in near-quantitative agreement with the experi- contributions of a much larger variety of PAHs.
mental spectra, both with respect to the frequen- To advance the interpretation and characteriza-
cies of vibrational bands and their intensities. tionofthesedata,wethereforeextendourexperi-
The shape of the 3-µm band recorded in as- mentalandtheoreticalstudiestoawidervarietyof
tronomical observations has been found to vary condensed and irregular isomers containing up to
within the same astronomical object and between fiverings(phenanthreneC H ,benz[a]antracene
14 10
different astronomical objects. To account for C H , chrysene C H , triphenylene C H ,
18 12 18 12 18 12
these differences, several explanations have been pyrene C H and perylene C H ). The goals
16 10 20 12
put forward (Sellgren et al. 1990; Tokunaga et al. of these studies are twofold. Firstly, we aim to
1991; van Diedenhoven et al. 2004). One of the understand how the effects of anharmonicity ob-
suggestions for classifying the shape of this band served for the linear PAHs are affected by geo-
istointerpretitasbeingassociatedwithemission metricalstructureandhowthisinturnaffectsthe
from two components (Song et al. 2003; Candian appearance of the IR absorption spectra in the 3-
et al. 2012) in which there are contributions from µm region. Secondly, we aim to uncover general
different groups of carriers at 3.28 and 3.30 µm. trends in band shapes that could provide spectral
Song et al. (2007) propose that these two compo- signatures that would allow for a much more de-
nents originate from groups of PAHs with differ- tailed description of the contribution of different
ent sizes, finding support for this in the labora- PAHs.
toryhigh-temperaturegas-phasestudiesofpyrene
(C H ) and ovalene (C H ) for which a blue 2. Methods
16 10 32 14
shiftofthe3-µmbandwasobserveduponincreas-
2.1. Experimental techniques
ing the size of the PAH (Joblin et al. 1995). An-
otherfactorthathasbeensuggestedtocontribute
IR spectra of cold and isolated molecules were
totheapparenttwo-componentappearanceofthe
obtained in molecular beam setup described in
emission are differences in the peripheral struc-
Smolarek et al. (2011). In these experiments the
ture of different PAHs, in particular steric effects
sample of interest was placed in an oven attached
as occurring for hydrogen atoms at so-called bay-
to a pulsed valve (General Valve). Sufficient va-
sites (Candian et al. 2012). The influence of the
por pressure was found to be obtained when the
edge structure has been investigated by means
samplewasheatedtotemperaturesslightlyhigher
of harmonic DFT calculations for large species
than its melting point. Subsequent pulsed expan-
(Bauschlicheretal.2009,2008)butsystematicex-
sion with 2 bars of argon as carrier gas and a typ-
perimental high-resolution studies are notoriously
ical opening time of 200 µs then led to supersonic
lacking. Similarly,ithasbeenfound(Geballeetal.
cooling of the sample molecules.
1989) that next to the prominent emission band
Theground-statevibrationalmanifoldofPAHs
at 3.29 µm, a broad plateau is present that spans
was probed by UV-IR ion dip spectroscopy. To
the 3.1-3.7 µm region and has been indicated as
this purpose two-color resonance enhanced mul-
the 3-µm plateau. It has been speculated that
tiphoton ionization followed by mass-selected ion
thisplateaumightinpartderivefromanharmonic
detection was used to generate an ion signal. In
couplings to vibrational combination levels (Alla-
these experiments PAHs were electronically ex-
3
cited by fixing a frequency-doubled pulsed dye the B9-71 functional (Hamprecht et al. 1998),
laser (Sirah Cobra Stretch) on a) the S (1A ) and the TZ2P basis set (Dunning 1971), all of
1 1
← S (1A ) 0 - 0 transition at 29329.8 cm−1 for which has been found to give the best perfor-
0 1
phenanthrene(Amiravetal.1984);b)theS (1A(cid:48)) mance on organic molecules (Boese & Martin
1
← S (1A(cid:48)) 0 - 0 transition at 26534.4 cm−1 for 2004; Can´e et al. 2007). The SP16 calculations
0
benz[a]anthracene (Wick et al. 1993); c) the S utilize the quadratic, cubic, and quartic force
1
(1B ) ← S (1A ) 0 - 0 transition at 28194.3 constants calculated by Gaussian09 and trans-
u 0 g
cm−1 for chrysene (Zhang et al. 2012); d) the S formed into Cartesian derivatives as the input for
1
(1B ) ← S (1A ) 0 - 0 transition at 27210.9 SPECTRO, which then performs its own vibra-
2u 0 g
cm−1 for pyrene (which is close to but not ex- tional second-order perturbation (VPT2) analysis
actlythesameasthepreviouslyreportedvalueby (Mackie et al. 2015b,a). SP16 treats the polyads
Ohta et al. (1987); e) the S (1A(cid:48)) ← S (1A(cid:48)) ofmultiplesimultaneousresonances(modesfalling
1 1 0 1
transition to the 249 (29867) cm−1 (1e) level in within 200 cm−1 of each other) and allows for
the excitedstate oftriphenylene (Harthcock et al. theredistributionofintensityamongtheresonant
2014; Kokkin et al. 2007); and f) the S (1B ) modes,whichisafunctionnotcurrentlyemployed
1 3u
← S (1A ) 0 - 0 transition at 24067.4 cm−1 for in Gaussian09. A more thorough account of the
0 g
perylene (de Vries et al. 1991). Molecules in elec- theoretical aspect of this work is given in a sep-
tronically excited states were ionized by an ArF arate publication (Mackie et al. 2016). Harmonic
excimerlaser(NeweksPSX-501)intemporalover- calculationsinthisworkwerescaledinordertobe
lap with the electronic excitation laser. compared with experimental data using a scaling
Prior to the pump and ionization laser beams factor (sf) of 0.961, while SP16 output does not
needed to generate this signal, an IR laser beam require scaling and can be compared directly.
with a linewidth of 0.07 cm−1 and a typical pulse
energy 1 mJ was introduced with a time delay of 3. Results
200 ns. The 3 µm beam was generated by differ-
In the following we will discuss IR-UV ion dip
ence frequency mixing of the fundamental output
spectra of phenanthrene, benz[a]antracene, chry-
of a dye laser (Sirah Precision Scan with LDS798
sene, triphenylene, pyrene and perylene. In the
dye) and the 1064 nm fundamental of Nd:YAG
first instance, we will present these spectra and
laser(SpectraPhysicsLab190)inaLiNbO crys-
3 compare them where possible with previous stud-
tal. Resonant excitation of vibrational levels was
ies, highlighting only salient features and differ-
observed by a decrease of the ion signal due to
ences between the various PAHs. We will make
depletionofthegroundstatepopulation,allowing
extensive comparisons with the results of theo-
for recording of IR absorption spectra. Such IR
retically predicted spectra. A more detailed dis-
spectra were recorded between 3.17 and 3.40 µm
cussion of these calculations and the assignment
(2950 and 3150 cm−1). With the present S/N ra-
of each of the observed bands in the experimen-
tios, no other IR bands could be observed outside
tal spectra will be presented in a separate study
this range.
(Mackie et al. 2016). Subsequently, we will make
aglobalcomparisonbetweenthespectra, anddis-
2.2. Computational methods
cuss their implications for extrapolating to the 3-
Two types of theoretical IR spectra were pro- µm region of larger PAHs. Finally, we will con-
duced: the standard harmonic vibrational ap- sider the current interpretation of astronomical
proach implemented in Gaussian09 (Frisch et al. PAH data in the light of the present results.
2009), and an anharmonic vibrational approach
that employs both Gaussian09 and a locally mod- 3.1. Phenanthrene
ified version of the program SPECTRO (Mackie
The 3-µm absorption spectrum of phenan-
et al. 2015a; Gaw et al. 1996), referred to in
threneisshowninFigure1,bottompanel. Intotal
this work as G09-h and SP16 calculations re-
twenty-threeexperimentalbandswithalinewidth
spectively. Both G09-h and SP16 calculations
2.5-4.5 cm−1 are observed. The positions and
start from DFT calculations that employ a sim-
strengths of these bands are reported in Table 1.
ilar integration grid as in Boese & Martin (2004),
4
The most active region (where the bands are at mum intensity at 3049.8 cm−1. The experimental
least 40% intensity of the dominant band) is lo- spectrum shows fourteen relatively narrow bands
catedbetween3030and3090cm−1 withthemost with a linewidth of 1.7-3.5 cm−1 whose positions
intense transition at 3065.3 cm−1. This region are given in table 1. This table also contains the
is accompanied by two weak bands in the low- positions of the ten bands reported previously by
energy region and five overlapping weak bands in Huneycutt et al. (2004), which agree well with
the high-energy wing. Table 1 also provides the the present observations (deviations less than 1
values of bands reported previously in the CRDS cm−1). Our S/N ratio, on the other hand, allows
studies of Huneycutt et al. (2004). Due to better us to identify weak bands at 3044, 3087.8, and
cooling conditions in the present experiments our 3096cm−1 thatcouldpreviouslynotbediscerned,
absorption spectrum shows bands that are more butdoesnotconfirmthepreviouslyreportedband
narrow. As a result, nine additional bands can at 3083.9 cm−1. Due to the D symmetry of
2h
be discerned, although we do not find evidence pyrene, only five IR active CH-stretch modes are
for the weak band previously reported at 3025.3 expected in the harmonic approximation (Fig.2,
cm−1. Overall, the agreement between the two top panel), which is clearly at odds with the ex-
sets of data is good with the largest deviation for periment. Indeed, the polyad VPT2 anharmonic
most of the bands not exceeding 2 cm−1. calculations (Fig.2, middle) are required to bring
Phenanthrene belongs to the C point group. experiment and theory into better agreement.
2ν
Within the harmonic approximation (Fig. 1, top
3.3. Benz[a]anthracene
panel) only six modes are therefore IR active in
the 3-µm region. The experimental spectrum, on TheIRabsorptionspectrumbenz[a]anthracene
the other hand, displays twenty-three bands and inthe2950-3150cm−1 regionisshowninFigure3
is a clear indicator of the failure of the harmonic together with the spectra predicted by harmonic
approximation. We conclude that, similar to lin- andanharmoniccalculations. Themainspectrum
ear PAHs (Maltseva et al. 2015), the CH-stretch feature covers quite a large frequency range and
region is dominated by Fermi resonances. Pre- lacks the narrow bands observed for the other
viously, we showed (Mackie et al. 2015b) that in PAHs reported here. Instead a broad structure
order to obtain good agreement between exper- between 3030 to 3090 cm−1 with the highest in-
imental and predicted spectra, VPT2 treatments tensity at 3063.8 cm−1 is seen. In this structure
arerequiredthattakeintensitysharingandpolyad only five bands can be distinguished clearly due
resonances as implemented in SP16 into account. to the overlap of a large number of blended tran-
Withsuchcalculations,theredistributionofinten- sitions in the present resolution.
sities over fundamental and combination bands –
The presently obtained spectrum is the first
in this case overtones are not accessible due to
high-resolution gas-phase IR spectrum that has
symmetry restrictions – of the same symmetry
been reported for benz[a]anthracene. We there-
leads to observable activity of many additional
fore compare in Table 1 the line positions as de-
bands. Comparison between the 3-µm absorp-
rived from Figure 3 with the Matrix Isolation
tion spectrum of phenanthrene predicted by SP16
Spectroscopy (MIS) data of Hudgins & Sandford
and convolved with a 1 cm−1 Gaussian line shape
(1998b). Wefindlargerdeviationsofupto3cm−1
(Fig.1, middle panel) with the experimentally ob-
which likely find their origin in matrix-induced
tained spectrum indeed shows good agreement.
perturbations, difficulties in determination of the
center of bands due to their shape (as occurs for
3.2. Pyrene
themostintenseband),andthenoiselevel(asoc-
Figure2(bottompanel)displaystheIRabsorp- curs for the weak bands). Interestingly, our spec-
tion spectrum of pyrene in the 2950-3150 cm−1 trum does not give an indication for the presence
region as measured in the present study together ofabandat3119.5cm−1 thatwasreportedinthe
with spectra predicted by G09-h and SP16 cal- MIS studies, possibly originating from the lack of
culations. Compared to phenanthrene, the dom- mass selection in the MIS experiments.
inant vibrational activity is observed in a more The large frequency range over which vibra-
compactregionof3045-3065cm−1 withthemaxi- tional activity is observed is in line with the rel-
5
atively low symmetry of the molecule (C ). As a 3.5. Triphenylene
s
result,alltwelveCH-stretchmodesareformallyIR
TriphenylenecompletestheseriesofPAHswith
active,althoughtheharmoniccalculationspredict
four aromatic rings. For this molecule no high-
that only six of them have appreciable intensity
resolutiongas-phasevibrationalspectrahavebeen
(Fig.3, top). The SP16 calculations, on the other
reportedpreviously. The3-µmbandrecordedhere
hand,matchtheexperimentalspectrumverywell.
using UV-IR ion dip spectroscopy is depicted in
They predict activity of a plethora of vibrational
Figure5(bottompanel). Activityisobservedover
transitionsasisclearwhenthestickspectrumun-
a relatively small range of 3030-3105 cm−1 with
derlying the green trace in Fig. 3 (middle panel)
threestrongfeaturesat3040,3075and3100cm−1
is inspected, and thereby confirm the conclusions
and the strongest band at 3101.8 cm−1. Above
that the the broad appearance of the spectrum is
3105 cm−1, one band at 3143.2 cm−1 appears to
duetotheoverlappingofmanyanharmonicbands.
be present, there is no indication for bands be-
low 3025 cm−1. Overall, sixteen bands with an
3.4. Chrysene
irregular shape and a minimum linewidth of 2.9
Chrysene (C18H12) is a highly symmetric iso- cm−1 can be resolved. Considering their profile,
mer of benz[a]anthracene (C2h versus Cs). Its ex- it is likely that most of these bands actually con-
perimentally obtained and theoretically predicted sist of several overlapping transitions. Previous
IR absorption spectra are shown in Figure 4. In MISstudies(Hudgins&Sandford1998b)compare
contrast to benz[a]anthracene, the experimental well with the present data (see Table 2) with dif-
spectrum displays a larger number of resolved ferences in band positions not exceeding 2 cm−1.
bands that have line widths which range from 3-7 In the present study significantly more bands are
cm−1. Remarkably, major vibrational activity is observed than in the MIS studies, and the MIS
found in the 3015-3115 cm−1 range, which is even 3116.9 cm−1 band is not confirmed.
larger than for benz[a]anthracene, with the most
Triphenylene belongs to the D symmetry
3h
intense band at 3063 cm−1. In this respect, it is
point group and because of this high symmetry
interesting to note the presence of bands above
only four pairs of doubly degenerate CH-stretch
3090 cm−1 where no activity was observed for
modes are IR active in the harmonic approxi-
benz[a]anthracene.
mation. As Gaussian 09 can not perform the
Table 2 reports the line positions of the fif- anharmonic analysis of molecules with D sym-
3h
teen bands that can be distinguished and com- metry, the calculation was done on triphenylene
paresthemwithpreviouslyreportedMISpositions with a reduced C symmetry caused by a small
2v
(Hudgins & Sandford 1998b). This comparison perturbation of the masses of two opposing car-
shows deviations less than 2 cm−1 caused mainly bon atoms, from 12 to 12.01 au (see Mackie et al.
by the inability to accurately identify line posi- (2016) for further details).
tionsofcloselyingbandsundertherelativelylow-
Under such conditions a spectrum is predicted
resolution conditions of the MIS experiment. The
(Fig. 5, top)withonlytwostrongbands, whichis
gas-phase data furthermore identify eight previ-
clearly at odds with the experiment. SP16 calcu-
ously unreported transitions, but does not show
lations incorporating anharmonicity, on the other
evidence for the MIS band at 3134.9 cm−1. Sym-
hand, redistribute the intensities among funda-
metry considerations lead to the conclusion that
mentals and combination bands with equal sym-
at most six CH-stretch modes could be IR-active
metries and lead to a predicted absorption spec-
(Fig.4, top panel) and that they are localized in a
trum that resembles the experimentally observed
relatively narrow frequency range. SP16 calcula-
one(Fig.5,middlepanel). Nevertheless,adetailed
tions (Fig.4, middle panel) once again emphasize
comparison of the two spectra does lead to the
the important role of anharmonicity and Fermi
conclusion that in the regions above 3105 and be-
resonances, and give rise to a predicted spectrum low 3025 cm−1 quite a larger activity is predicted
that is in good agreement with the experiment.
by the calculations than observed experimentally.
In fact, similar observations can also be made
for chrysene (regions above 3110 and below 3020
6
cm−1) and to some extent for benz[a]anthracene parisons with the experimental spectra. Further
(region above 3090 cm−1) although in these cases studies on the functional/basis set dependence of
the differences are not as obvious as in the case of the quartic force field of large molecules such as
triphenylene. perylene would thus be of interest, but fall out-
side the scope of the present studies.
3.6. Perylene
4. Discussion
Perylene is the most condensed five-aromatic
ring PAH system. The experimental IR absorp-
The high-resolution IR absorption spectra of
tion spectrum of this molecule in the 3-µm re-
the presently studied set of PAHs together with
gion is shown in the bottom panel of Figure 6 to-
those of the linear acenes studied previously
gether with predicted spectra at the G09-h level
(Maltseva et al. 2015) provide a comprehensive
(top panel). The spectrum is quite compact and
view on key factors that determine the appear-
dominated by a group of bands located in 3056-
ance of the 3-µm band in PAHs. The number
3070 cm−1 with the most intense transition oc-
of CH-oscillators obviously controls the number
curring at 3063.8 cm−1. At the high-energy side
of normal modes, and this clearly implies that
a weak, sharp band is observed at 3118 cm−1 and
compact species always have less active modes in
a broad structure that appears to consist of three
the 3-µm region than more extended molecules of
overlappingbandsat3092.8,3095.9,3098.2cm−1.
the same symmetry and with the same number
Overall, nine bands with a linewidth of (cid:62)3 cm−1
of rings. However, from the comparison of 3-µm
are reported in Table 2. Perylene has previously
bands of PAHs with different molecular structure
been studied with CRDS (Huneycutt et al. 2004),
it becomes clear that the frequency region over
butthespectrumreportedatthattimeclearlyhas
which activity is observed and the distribution of
a lower S/N ratio. This is most likely the reason
intensity over this region is not as dependent on
that a comparison between the two spectra is not
thenumberofCH-oscillatorsasonemightexpect.
asfavorableasfortheothermolecules. Forexam-
For example, four-ring pyrene consists of six-
ple, we do not find indications for the previously
teen carbons and ten hydrogens and its 3-µm
reported bands at 3044.9 and 3070.4 cm−1 and
bands looks quite similar to anthracene (Fig.7),
cannot confirm the band at 3022 cm−1. More-
anothermoleculewithtenCH-oscillatorsandD
over, our cooling conditions enable us to conclude 2h
symmetry. Both 3-µm bands are compact, the
thatthebandsreportedat3096.1cm−1and3065.3
most active region occurring in a range of 22
cm−1 (CRDS) consist of three and two bands, re-
and 30 cm−1, respectively. However, a notice-
spectively. Perylene possesses D symmetry and
2h able blue-shift of the strongest transition of an-
according to the harmonic approximation only 6
thracene (3071.9 cm−1) with respect to pyrene
CH-stretch modes are IR active. The other ob-
(3049.8 cm−1) is observed. Phenathrene is an iso-
servedbandsoriginatefromanharmonicactivities
merofanthracene,butdespitejustasmallchange
– i.e., resonances.
in molecular structure, its 3-µm band is very
Unfortunately, it was not possible to perform
different (Fig.7). The most active region spans
ananharmonicanalysisonperyleneasGaussian09 60 cm−1 with the most intense band at 3065.3
wasunabletoproducereasonablecubicandquar- cm−1. These differences likely originate from the
ticforceconstants. Thoseunreasonableforcecon-
nonlinear structure of phenanthrene. Significant
stants lead to a significant overestimation of the
changes occur in the periphery of the molecule.
anharmonic corrections of band positions, with
Anthracenehastwosolo-hydrogensandtwoquar-
manybandspredictedtooccurathigherenergies,
tets. Phenanthrene, on the other hand, has two
which is contrary to what is expected. It would
duo hydrogens instead of two solos, and also two
appearthattheseproblemsdependonthelevelof
quartets, but conversely the quartets each have a
theory that is used since calculations on perylene
hydrogen in the bay-region (Fig. 8). As a result,
using the B3LYP functional with a 4-31G basis
steric effects increase the vibrational frequencies
set lead to cubic and quartic force constants that
of these CH-oscillators (Bauschlicher et al. 2009).
seem much more reasonable. However, this level
Interestingly, Figure9showsthatthe3-µmbands
of theory is too low to allow for meaningful com-
7
of phenanthrene resembles to a large extent that the spectrum undergoes changes. Chrysene shows
of benz[a]anthracene. The most intense transi- prominent high-energy features in the 3090-3120
tions of phenanthrene and benz[a]anthracene are cm−1 regionwheretetracenehasconsiderablyless
at3065.3and3063.8cm−1,themostactiveregions activity (Fig.10). We conclude that this activity
are 3030-3090 and 3035-3097 cm−1, respectively. should be attributed to bands involving modes
This can be rationalized on the basis of their sim- with considerable contributions of hydrogens in
ilar peripheral structure, the only difference be- thebayregionasthesehaveintrinsicallyhighervi-
tween the two molecules being the additional two brational frequencies because of steric hindrance.
solohydrogensinbenz[a]anthracene,whichdonot The conclusion is supported by the SP16 calcula-
contribute significantly to the spectrum. tions. It is found computationally that all reso-
The 3-µm absorption of pyrene looks very sim- nancesinthisregioninvolvetheCH-stretchmode
ilar to the absorption of the five-ring PAH with ofchrysenelocatedinabayregion. Allhydrogens
twelvehydrogens-perylene(Fig.9). Themostac- of triphenylene are incorporated into three quar-
tive modes of perylene are concentrated in a nar- tets with half of them in bay-sites. In contrast
row frequency region of 15 cm−1. This relatively totheotherPAHswithquartets,triphenylenehas
small range follows quite nicely from the molecu- the most intense band at 3101.8 cm−1 while for
lar and, in particular, the peripheral structure of the other compounds it is found between 3063-
perylene. All hydrogens in perylene come as trios 3066cm−1 (Fig.10). Thisisacleardemonstration
and thus contribute to IR absorption at similar of the presence of bay-site hydrogens. The band
frequencies. Perylene and pyrene indeed have a at 3076.2 cm−1 most likely dominantly involves
similar band at 3064 cm−1 which is attributed to asymmetrically stretching of bay-hydrogens. The
modes localized in both cases in trio hydrogens. 3030-3060cm−1 regioncannicelybeexplainedby
Pyrene does not possess bay-hydrogens, but pery- anharmonic activity (Mackie et al. 2016). Such a
lene does. As confirmed by the calculations this comparisonclearlyshowsthatvibrationsinvolving
explains the differences in activity in the high- hydrogens in bay-site are blue-shifted.
frequency regions of the spectra of the two com- The symmetry of the molecule controls the
pounds. number of IR-allowed transitions. These transi-
From the subsequent discussion (vide infra) it tions are the source of intensity for combination
appears that molecules with the same type of hy- bands of the same symmetry that normally would
drogens (solos, duos, trios, or quartets) display be ’dark’ but can acquire intensity through Fermi
activity in a similar frequency range. Moreover, resonance. When comparing isomers with differ-
fromthecomparisonofourexperimentwith DFT ent symmetries, for example anthracene and phe-
theory, we suggest that the frequencies of normal hanthrene, the isomer with a higher symmetry
modes that primarily involve motion of the same (anthracene) is thus expected to show a smaller
type of hydrogens are restricted to rather narrow numberoffundamentalandcombinationbandsas
frequencyregionswithfrequenciesthatincreasein is indeed confirmed by our experiments (Fig.7).
the order of solos, duos, trios, to quartet (Mackie Therefore,moleculeswithlowdegreeofsymmetry,
et al. 2016). likebenz[a]anthracene,demonstratebroadabsorp-
tion because of the overlap of anharmonic bands
Anothereffectinducedbysterichindranceasso-
(Fig.3).
ciatedwithbayregionsisbestillustratedbycom-
paring the 3-µm absorption of tetracene with its Another important conclusion that can be
isomers in Fig.10. Benz[a]anthracene has a pair drawn from the present experiments concerns the
of hydrogens (one is from a quartet and another frequency range over which anharmonicity is ac-
one is a solo hydrogen) in a bay-site. Its 3-µm tive in the 3-µm region. We have acquired high-
bandhasamaximumat3063.8cm−1,closetothe resolution IR absorption spectra for PAHs with
mostintensetransitionoftetracene(3061.1cm−1) two to five rings in all possible geometrical ar-
and major activity over 3035-3097 cm−1. While rangements, but for all these species vibrational
thestrongesttransitionofchrysene(3063cm−1)is activity quickly disappears below, roughly speak-
notaffectedbythestructuralchangesandremains ing, 3000 cm−1 (above 3.33 µm). This conclusion
very close to tetracene, the high-energy part of is supported by our SP16 calculations that also
8
show very limited activity in this region. More- et al.2007),appearstobeonlyofsecondaryinflu-
over, in the SP16 calculations an important pa- ence. Nevertheless, our results also unmistakably
rameter that controls the number of states to be demonstrate that activity in the high-frequency
included in resonanaces/polyads is the maximum range is not only induced by bay-hydrogens, but
energy separation of the states. In all our cal- also occurs for PAHs without bay-sites. The finer
culations we find that a maximum separation of detailsofthispartofthespectrumthusarethere-
200 cm−1 is more than adequate. On the basis sult from a subtle interplay between the effects of
of the scaled harmonic calculations in which the sterichindranceasencounteredforthebayhydro-
lowest frequencies occur around 3025 cm−1 one gens and anharmonicities. This implies that one
can then conclude that the maximum wavelength should be cautious in directly relating the ratio of
rangeoverwhichcombinationbandactivitymight the intensities of the two bands to abundances of
be observed extends only up to 2825 cm−1 (3.54 compact and extended PAH structures.
µm), and this number is most likely even an over- Another secondary feature that has attracted
estimate. Increasing the size of a PAH will thus considerable attention is the 3-µm plateau span-
increasethedensityof‘dark’statesthatmightpo- ning the 3.1-3.7 µm (2700-3200 cm−1) region (Al-
tentiallycoupletothe‘bright’zeroth-orderstates, lamandola et al. 1989; Geballe et al. 1989). Our
but the present study strongly suggests that this spectrashowthattheintensityinthehigh-energy
increased density has a very limited effect on the part of this plateau is derived from Fermi res-
rangeoftheregionoverwhichvibrationalactivity onances between fundamental CH-stretch transi-
is observed. tions and the plethora of combination bands that
are present in this region. However, such an ex-
5. Astrophysical implications planation, which previously has also been put for-
ward as a possible cause for the lower-energy side
The present study confirms the dominant role
of the plateau (Allamandola et al. 1989), is not
of anharmonicity in determining the shape and
supported by the present results which show only
strength of the features in the CH-stretching re-
very limited activity in the region above 3.3 µm
gion. Predictions by harmonic calculations thus
(below3000cm−1). Above,wehavereasonedthat
are far from adequate for obtaining a fundamen-
alsoforlargerPAHswedonotexpectanincreased
tal understanding of how the 3-µm region reflects
activityinthisregion. Wethereforeconcludethat
the chemical composition and evolution of astro-
in this region IR absorption has another origin.
nomicalobjects, andfortryingtounderstandsec-
A final feature of interest is the 3.40-µm fea-
ondary features around the 3-µm feature. They
ture on the 3-µm plateau. Several explanations
alsoindicatethatinprincipletherearemanymore
have been put forward to account for its presence
leads available for identifying single PAH species
ranging from hot bands of aromatic CH-stretch
as intensity is distributed over many more transi-
transition υ = 2 → 1 shifted due to anharmonic
tions than only the fundamental CH-stretch tran-
effects (Barker et al. 1987) to CH-stretch modes
sitions.
in methylated (Joblin et al. 1996; Pauzat et al.
Oneofthesecondaryfeaturesthathasbeendis-
1999) and superhydrogenated (Bernstein et al.
cussed extensively in literature concerns the two-
1996; Steglich et al. 2013; Sandford et al. 2013;
component emission character of the 3-µm band
Wagneretal.2000)PAHs,butnoneofthesehave
(Song et al. 2003). Our results clearly show that
so far been confirmed. The overall picture that
bay-hydrogensinduceintensityatthehigh-energy
emanatesfromourhigh-resolutionstudies,involv-
side, and thus fully support the previous sugges-
ing in particular the role of anharmonicity, sug-
tion that the 3.28-µm emission band is associ-
gests that an explanation based on hydrogenated
ated with modes involving bay-hydrogens as oc-
andalkylatedPAHsisattractive. Methylatedand
curringinmoreextendedPAHs,whilethe3.30-µm
hydrogenated PAHs have been known to be IR
emission band derives from more compact PAH
active at much lower frequencies than the fully
structures (Candian et al. 2012). The size of the
aromatic systems and might thus contribute to
PAH, which also has been put forward as the pri-
the 3-µm plateau. Assuming that anharmonicity
marycauseforthetwo-componentemission(Song
plays a similar role in such alkylated PAHs, this
9
would then also explain the extended range of the Such studies are presently underway.
3-µm plateau which cannot be explained merely Our studies show that the vibrational activity
in terms of bare PAHs. To find further support in the CH-stretch fingerprint region is strongly
for such a conclusion we are presently performing linked to details of the molecular structure. It
high-resolutionIRabsorptionstudiesonanexten- is therefore not only different for molecules with
sive series of hydrogenated and alkylated PAHs. differentchemicalstructures,butalsofordifferent
isomers. We have demonstrated that the periph-
6. Conclusions eral structure of the molecules plays a dominant
role in the appearance of the 3-µm band. Studies
Inthisworkwehavepresentedmolecularbeam
in which the 3-µm region is correlated with the
IR absorption spectra of six condensed PAHs in
”periphery-sensitive” 9-15 µm region are thus of
the3-µmregionusingIR-UViondipspectroscopy.
significantinterestandcouldfurtherelucidatethe
Thepresentresultsandtheresultsonlinearacenes
exact composition of the carriers of these bands.
(Maltseva et al. 2015; Mackie et al. 2015b) show
that anharmonicity indeed rules the 3-µm region,
The experimental work was supported by The
with the fraction of intensity not associated with
Netherlands Organization for Scientific Research
fundamental transitions easily exceeding 50%.
(NWO).APacknowledgesNWOforaVIDIgrant
Anharmonicity-induced transitions are more the
(723.014.007). StudiesofinterstellarPAHsatLei-
rule than the exception and should explicitly be
den Observatory have been supported through
taken into account in the interpretation of astro-
the advanced European Research Council Grant
nomical data in the 3-µm region. A proper incor-
246976andaSpinozaaward. Computingtimehas
poration of resonances has been shown to yield
beenmadeavailablebyNWOExacteWetenschap-
predicted spectra that are in semi-quantitative
pen (project MP-270-13 and MP-264-14) and
agreement with the experiments. Such calcula-
calculations were performed at the LISA Linux
tions may therefore lead the way for furthering
cluster and Cartesius supercomputer (SurfSARA,
ourunderstandingontheinfluenceoflargerPAHs
Almere, NL). AC acknowledges NWO for a VENI
which are not amenable to similar experimental
grant (639.041.543). XH and TJL gratefully ac-
high-resolution studies.
knowledge support from the NASA 12-APRA12-
Our work shows that the observed abundance
0107 grant. XH acknowledges the support from
of combination bands is mostly concentrated in
NASA/SETI Co-op Agreement NNX15AF45A.
the low-energy part (≤ 3100 cm−1) of the CH-
Some of this material is based upon work sup-
stretchregionandoriginatesfromcombinationsof
ported by the National Aeronautics and Space
CC-stretchandCHin-planebendingmodes. This
Administration through the NASA Astrobiology
anharmonic activity can be partly responsible for
Institute under Cooperative Agreement Notice
the 3-µm plateau observed by astronomers al-
NNH13ZDA017C issued through the Science Mis-
thoughbothexperimentandtheoryputintoques-
sion Directorate
tion a scenario in which the plateau is solely at-
tributedtoFermiresonancesbetweenfundamental
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10