Electronic Spectroscopy of Polycyclic Aromatic Hydrocarbons in solid para-Hydrogen
I. Weber1 and Y.-P. Lee1,21

Dept. Applied Chemistry and Center for Emergent Functional Matter Science, National Yang-Ming Chiao Tung
University, Taiwan
2Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, Taiwan

Polycyclic aromatic hydrocarbons (PAH) and their heteroatom-substituted, protonated,hydrogenated, and cationic derivatives are commonly considered as the carriers of theunidentified infrared (UIR) bands and as promising candidates for the carriers of the diffuse
interstellar bands (DIB), [1] absorption bands in the UV/VIS to near-IR region. However,despite the large number of known DIB (well over 200 up to today), only thebuckminsterfullerene cation C60+ has been confirmed as the carrier of two DIB at 9577.5 and9632.7 Å on the basis of its laboratory spectrum. [2,3]Para-hydrogen (p-H2) has frequently been employed as a matrix host for IR spectroscopy ofPAH, PANH, and their protonated and hydrogenated derivatives; the IR absorption spectra
exhibit only small shifts relative to the gas-phase, in line with the “softness” of solid p-H2.
Over the past years, we have recorded the dispersed fluorescence and fluorescence excitation
spectra of various PAH with 10 to 42 carbon atoms as well as a small number of protonatedand hydrogenated PAH and PANH, to assess the properties of p-H2 as a matrix host forelectronic spectroscopy and its potential to provide laboratory reference spectra forcomparison to observations. We find a generally good agreement with previously literature
data, indicating a rather small but consistent redshift of <100 cm−1 relative to the gas phasedue to the matrix environment.
In our contribution, we discuss advantages and disadvantages of p-H2 as a matrix host forelectronic spectroscopy by providing experimental data and spectra simulated based on
quantum-chemical calculations for three examples: the 1-hydronaphthyl radical (1-C10H9),
which has been extensively studied in the gas phase and in solid Ne, and the two large PAH
peri-hexabenzocoronene (peri-HBC, C42H18) and ovalene (C32H14). Our work on 1-C10H9
demonstrates the good agreement of the p-H2 data with previous work and the comparablyeasy accessibility of hydrogenated (and protonated) PAH in p-H2, but also shows the stronger
influence of p-H2 on spectral line positions compared to Ne. We observed the D1–D0 origin at
18881 cm−1 giving a matrix shift of 68 cm−1; for Ne, a blue shift of ~13 cm−1 can be inferredfrom the work of Garkusha et al. [4] Peri-HBC has rarely been studied before. We provide afirst full assignment of the vibrational structure of the fluorescence emission and excitation
spectra centered around the S1–S0 origin band located ~22075 cm−1 in solid p-H2. Ovalene has
been discussed as potential DIB carrier and a system exhibiting anomalous fluorescence fromhigher excited singlet states. Combining experiment and theory, we found, however, that thelowest excited singlet state (S1) was previously assigned erroneously; we report thefluorescence emission and excitation spectra as well as fluorescence lifetimes of the true S1state.
References

[1]
[2]
[3]
[4]

A. G. G. M. Tielens, 2008, Ann. Rev. Astron. Astrophys. 46, 289.
E. K. Campbell, M. Holz, P. & J. P. Maier, 2015, Nature 523, 322.
E. K. Campbell, M. Holz, J. P. Maier, P. Gerlich, G. A. H. Walker & D. Bohlender, 2016, ApJ 822, 17.
I. Garkusha, A. Nagy, J. Fulara, M. F. Rode, A. L. Sobolewski & J. P. Maier, 2013, JPCA 117, 351.