The 700-1500 cm⁻¹ region of the S1 (ùB2) state of toluene studied with resonance-enhanced multiphoton ionization (REMPI), zero-kinetic-energy (ZEKE) spectroscopy, and time-resolved slow-electron velocity-map imaging (tr-SEVI) spectroscopy.

We report (nanosecond) resonance-enhanced multiphoton ionization (REMPI), (nanosecond) zero-kinetic-energy (ZEKE) and (picosecond) time-resolved slow-electron velocity map imaging (tr-SEVI) spectra of fully hydrogenated toluene (Tol-h8) and the deuterated-methyl group isotopologue (α3-Tol-d3). Vibrational assignments are made making use of the activity observed in the ZEKE and tr-SEVI spectra, together with the results from quantum chemical and previous experimental results. Here, we examine the 700-1500 cm(-1) region of the REMPI spectrum, extending our previous work on the region ≤700 cm(-1). We provide assignments for the majority of the S1 and cation bands observed, and in particular we gain insight regarding a number of regions where vibrations are coupled via Fermi resonance. We also gain insight into intramolecular vibrational redistribution in this molecule.

Interaction of the NO 3pπ Rydberg state with Ar: potential energy surfaces and spectroscopy.

We present the experimental and simulated (2+1) REMPI spectrum of the C(2)Π state of the NO-Ar complex, in the vicinity of the 3p Rydberg state of NO. Two Rydberg states of NO are expected in this energy region: the C(2)Π (3pπ) and D(2)Σ(+) (3pσ) states, and we concentrate on the former here. When the C(2)Π (3pπ) state interacts with Ar at nonlinear orientations, the symmetry is lowered to C(s), splitting the degeneracy of the (2)Π state to yield C((2)A") and C((2)A') states. For these two states of NO-Ar, we calculate potential energy surfaces using second order Møller-Plesset perturbation theory, exploiting a procedure to converge the reference Hartree-Fock wavefunction to describe the excited states, the maximum overlap method. The bound rovibrational states obtained from the surfaces are used to simulate the electronic spectrum, which is in excellent agreement with experiment, providing assignments for the observed spectral lines from the calculated rovibrational wavefunctions.

Vibrations of the low energy states of toluene (X̃ (1)A1 and à (1)B2) and the toluene cation (X̃ (2)B1).

We commence by presenting an overview of the assignment of the vibrational frequencies of the toluene molecule in its ground (S0) state. The assignment given is in terms of a recently proposed nomenclature, which allows the ring-localized vibrations to be compared straightforwardly across different monosubstituted benzenes. The frequencies and assignments are based not only on a range of previous work, but also on calculated wavenumbers for both the fully hydrogenated (toluene-h8) and the deuterated-methyl group isotopologue (α3-toluene-d3), obtained from density functional theory (DFT), including artificial-isotope shifts. For the S1 state, one-colour resonance-enhanced multiphoton ionization (REMPI) spectroscopy was employed, with the vibrational assignments also being based on previous work and time-dependent density functional theory (TDDFT) calculated values; but also making use of the activity observed in two-colour zero kinetic energy (ZEKE) spectroscopy. The ZEKE experiments were carried out employing a (1 + 1(')) ionization scheme, using various vibrational levels of the S1 state with an energy <630 cm(-1) as intermediates; as such we only discuss in detail the assignment of the REMPI spectra at wavenumbers <700 cm(-1), referring to the assignment of the ZEKE spectra concurrently. Comparison of the ZEKE spectra for the two toluene isotopologues, as well as with previously reported dispersed-fluorescence spectra, and with the results of DFT calculations, provide insight both into the assignment of the vibrations in the S1 and D0(+) states, as well as the couplings between these vibrations. In particular, insight into the nature of a complicated Fermi resonance feature at ∼460 cm(-1) in the S1 state is obtained, and Fermi resonances in the cation are identified. Finally, we compare activity observed in both REMPI and ZEKE spectroscopy for both toluene isotopologues with that for fluorobenzene and chlorobenzene.

Spectroscopy of the à state of NO-alkane complexes (alkane = methane, ethane, propane, and n-butane).

We have recorded (1+1) resonance-enhanced multiphoton ionization spectra of complexes formed between NO and the alkanes: CH(4), C(2)H(6), C(3)H(8), and n-C(4)H(10). The spectra correspond to the à ← X̃ transition, which is a NO-localized 3s ← 2pπ* transition. In line with previous work, the spectrum for NO-CH(4) has well-defined structure, but this is only partially resolved for the other complexes. The spectra recorded in the NO(+)-alkane mass channels all show a slowly rising onset, followed by a sharp offset, which is associated with dissociation of NO-alkane, from which binding energies in the X̃ and à states are deduced. Beyond this sharp offset, there is a further rise in signal, which is attributed to fragmentation of higher complexes, NO-(alkane)(n). Analysis of these features allows binding energies for (NO-alkane)···alkane to be estimated, and these suggest that in the NO-(alkane)(2) complexes, the second alkane molecule is bound to the first, rather than to NO. Calculated structures for the 1:1 complexes are reported, as well as binding energies.