High resolution electron impact studies of electric dipole-forbidden states of benzene

Electron impact electronic excitation of benzene: Theory and experiment

We report experimental differential cross sections (DCSs) for electron impact excitation of bands I to V of benzene at incident energies of 10, 12.5, 15, and 20 eV. They are compared to calculations using the Schwinger multichannel method while accounting for up to 437 open channels. For intermediate scattering angles, the calculations reveal that the most intense band (V) emerges from surprisingly similar contributions from all its underlying states (despite some preference for the dipole-allowed transitions). They further shed light on intricate multichannel couplings between the states of bands I to V and higher-lying Rydberg states. In turn, the measurements support a vibronic coupling mechanism for excitation of bands II and IV and also show an unexpected forward peak in the spin-forbidden transition accounting for band III. Overall, there is decent agreement between theory and experiment at intermediate angles and at lower energies and in terms of the relative DCSs of the five bands. Discrepancies between the present and previous experiment regarding bands IV and V draw attention to the need of additional experimental investigations. We also report measured DCSs for vibrational excitation of combined C-H stretching modes.

Evaluated electron scattering cross section dataset for gaseous benzene in the energy range 0.1-1000 eV

In this study, a complete and self-consistent cross section dataset for electron transport simulations through gaseous benzene in the energy range 0.1-1000 eV has been critically compiled. Its reliability has been evaluated through a joint experimental and computational procedure. To accomplish this, the compiled dataset has been used as input for event-by-event Monte Carlo simulations of the magnetically confined electron transport through gaseous benzene, and the simulated transmitted intensity has been compared with the experimental one for different incident energies and benzene gas pressures.

Vacuum ultraviolet photoabsorption spectroscopy of crystalline and amorphous benzene

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 ¹B_2u, ¹B_1u and ¹E_1u states, redshifted by 0.05, 0.25 and 0.51 eV respectively with respect to the gas phase. The symmetry forbidden ¹B_2u ← ¹A_1g and ¹B_1u ← ¹A_1g transitions exhibit vibronic structure due to vibronic coupling and intensity borrowing from the allowed ¹E_1u ← ¹A_1g transition. Additionally the ¹B_2u ← ¹A_1g 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 ¹B_1u ← ¹A_1g 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 ³B_2u ← ¹A_1g, ³E_1u ← ¹A_1g and ³B_1u ← ¹A_1g 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.

Distortion dependent intersystem crossing: A femtosecond time-resolved photoelectron spectroscopy study of benzene, toluene, and p-xylene

The competition between ultrafast intersystem crossing and internal conversion in benzene, toluene, and p-xylene is investigated with time-resolved photoelectron spectroscopy and quantum chemical calculations. By exciting to S2 out-of-plane symmetry breaking, distortions are activated at early times whereupon spin-forbidden intersystem crossing becomes (partly) allowed. Natural bond orbital analysis suggests that the pinnacle carbon atoms distorting from the aromatic plane change hybridization between the planar Franck-Condon geometry and the deformed (boat-shaped) S2 equilibrium geometry. The effect is observed to increase in the presence of methyl-groups on the pinnacle carbon-atoms, where largest extents of σ and π orbital-mixing are observed. This is fully consistent with the time-resolved spectroscopy data: Toluene and p-xylene show evidence for ultrafast triplet formation competing with internal conversion, while benzene appears to only decay via internal conversion within the singlet manifold. For toluene and p-xylene, internal conversion to S1 and intersystem crossing to T3 occur within the time-resolution of our instrument. The receiver triplet state (T3) is found to undergo internal conversion in the triplet manifold within ≈100-150 fs (toluene) or ≈180-200 fs (p-xylene) as demonstrated by matching rise and decay components of upper and lower triplet states. Overall, the effect of methylation is found to both increase the intersystem crossing probability and direct the molecular axis of the excited state dynamics.

Ultrafast electron diffraction: Excited state structures and chemistries of aromatic carbonyls

The photophysics and photochemistry of molecules with complex electronic structures, such as aromatic carbonyls, involve dark structures of radiationless processes. With ultrafast electron diffraction (UED) of isolated molecular beams it is possible to determine these transient structures, and in this contribution we examine the nature of structural dynamics in two systems, benzaldehyde and acetophenone. Both molecules are seen to undergo a bifurcation upon excitation (S(2)). Following femtosecond conversion to S(1), the bifurcation leads to the formation of molecular dissociation products, benzene and carbon monoxide for benzaldehyde, and benzoyl and methyl radicals for acetophenone, as well as intersystem crossing to the triplet state in both cases. The structure of the triplet state was determined to be "quinoidlike" of pipi(*) character with the excitation being localized in the phenyl ring. For the chemical channels, the product structures were also determined. The difference in photochemistry between the two species is discussed with respect to the change in large amplitude motion caused by the added methyl group in acetophenone. This discussion is also expanded to compare these results with the prototypical aliphatic carbonyl compounds, acetaldehyde and acetone. From these studies of structural dynamics, experimental and theoretical, we provide a landscape picture for, and the structures involved in, the radiationless pathways which determine the fate of molecules following excitation. For completeness, the UED methodology and the theoretical framework for structure determination are described in this full account of an earlier communication [J. S. Feenstra et al., J. Chem. Phys. 123, 221104 (2005)].