Vacuum ultraviolet photoabsorption spectroscopy of crystalline and amorphous benzene

Abstract

We present the first high resolution vacuum ultraviolet photoabsorption study of amorphous benzene with comparisons to annealed crystalline benzene and the gas phase. Vapour deposited benzene layers were grown at 25 K and annealed to 90 K under conditions pertinent to interstellar icy dust grains and icy planetary bodies in our solar system. Three singlet-singlet electronic transitions in solid benzene correspond to the B, B and E states, redshifted by 0.05, 0.25 and 0.51 eV respectively with respect to the gas phase. The symmetry forbidden B ← A and B ← A transitions exhibit vibronic structure due to vibronic coupling and intensity borrowing from the allowed E ← A transition. Additionally the B ← A structure shows evidence of coupling between intramolecular vibrational and intermolecular lattice modes in crystalline benzene with Davydov crystal field splitting observed. The optically forbidden 0-0 electronic origin is clearly visible as a doublet at 4.69/4.70 eV in the crystalline solid and as a weak broadened feature at 4.67 eV in amorphous benzene. In the case of the B ← A transition the forbidden 0-0 electronic origin is only observed in crystalline benzene as an exciton peak at 5.77 eV. Thicker amorphous benzene samples show diffuse bands around 4.3, 5.0 and 5.4 eV that we tentatively assign to spin forbidden singlet-triplet B ← A, E ← A and B ← A transitions respectively, not previously reported in photoabsorption spectra of amorphous benzene. Furthermore, our results show clear evidence of non-wetting or 'islanding' of amorphous benzene, characterised by thickness-dependent Rayleigh scattering tails at wavelengths greater than 220 nm. These results have significant implications for our understanding of the physical and chemical properties and processes in astrochemical ices and highlight the importance of VUV spectroscopy.

A fibre-coupled UHV-compatible variable angle reflection-absorption UV/visible spectrometer

“…The calculated value of n is in good agreement with recent work which, using a He-Ne laser as described above, determined values for solid benzene ranging from 1.38 -1.47 (± 0.06). 20 This confirms the validity of using Harrick's method 31 to analyse our data in the UV/visible region. The determined value of n is lower than that of the liquid at 293 K, given as 1.501.…”

“…These are assigned to the benzene A1g transition centred at 255 nm. 20 At wavelengths longer than approximately 270 nm the absorption cross-section of benzene is negligible 46 and absorption features remain constant. The peak at 656 nm on the 46° spectrum is an artefact arising from the ratioing process of the raw data containing features from the light source output, 45 as seen in Figure 3.…”

“…15 Often a He-Ne laser is reflected off the surface producing interference fringes during deposition. These fringes are used to determine thickness, 10,11,13,14,[16][17][18][19][20] d, using the equation below (1):…”

Probing the interaction between solid benzene and water using vacuum ultraviolet and infrared spectroscopy

“…The VUV photoabsorption measurements were carried out at the ASTRID2 Synchrotron Facility, Aarhus, Denmark by attaching the portable system to the AU-UV beamline. 20 This work is a continuation of the work we published in Dawes et al 19 (referred to as AD17 from now on), so a detailed description of the experimental set-up and procedures can be found there. In summary, for the data presented in this paper, the VUV spectra were acquired in the range 112-340 nm with 0.05-1 nm wavelength step size depending on the width of the spectral features to be resolved.…”

“…As such UV photoabsorption spectroscopy is necessary to better understand and quantify the interactions of UV photons with such species. In our previous paper 19 we presented the results of vacuum ultraviolet spectra of pure amorphous and crystalline benzene, as a prototypical aromatic molecule, investigated under conditions relevant to that of ices in the interstellar medium and on planetary surfaces. In this paper we have extended our study to investigate the interaction of benzene and water ice layers and mixtures of different concentrations.…”