Benjamin Scharf

Variations of splittings of vibronic Jahn–Teller states in asymmetrically deuterated benzenes

Measurements of the Rydberg absorption spectrum converging to the first ionization threshold of asymmetrically deuterated benzenes are reported. Evidence of splittings of the 0 → 0 bands in the R′ series upon reduction of the nuclear symmetry to D2h or C2v by deuteration is revealed. No such effect is present in the R series. The 0 → 1 bands of ν18(e2g) in the R′ series exhibit a much smaller splitting. A theoretical analysis predicting all of the facets of these phenomena is presented. These observations enable an independent assignment of the R and R′ series previously obtained on the basis of splitting in the hot band structure of the R′ series due to quadratic Jahn–Teller activity of the out‐of‐plane mode ν20(e2u) which cannot be linearly J–T active. These results conform with the direct observations of splittings of Jahn–Teller degeneracy obtained in benzene Rydberg series converging to the second ionization threshold upon asymmetric deuteration.

The assignment of the benzene first-ionization rydberg spectrum via J-T splittings involving linearly inactive modes

Abstract In this paper we report the first identification of a Jahn-Teller (J-T) splitting (≈ 160 cm−1) arising via quadratic vibronic coupling in a mode which is non-active in the linear J-T coupling. This splitting has been detected in the hot-band structure of the out-of-plane mode v20(e2u) in the Rydberg spectrum of benzene converging to the first ionization limit. This observation enables us to resolve the long-standing controversy with respect to the assignment of the relevant Rydberg spectrum.

Novel effects in the laser induced fluorescence spectrum of C6F+6

The B 2A2u←X 2E1g(0) laser induced fluorescence excitation spectrum of the hexafluorobenzene cation in a free jet expansion has been measured from 21 500 to 22 580 cm−1. The spectrum displays a number of strong transitions involving the totally symmetric and the Jahn–Teller active vibrations. In addition, many weak transitions involving other vibrational modes are observed using high laser powers. These weak, interspersed transitions are readily detected because the ion is cooled in a supersonic expansion which results in sharp, well resolved lines. These weak transitions derive their intensities by a variety of mechanisms, the most novel of which depends on the presence of mixed cross‐quadratic nuclear potential terms in the ground (2E1g) state. It is shown that these terms permit the observation of transitions from the vibrationless level of the ground state to combination levels of the excited state involving two modes of different symmetries. The identification of transitions of this type provides t...

Enhanced two-photon transitions in molecules with permanent dipole moments

Abstract For radiative processes involving two photons of different color in the limit when one of the photon frequencies is small, we show that the rate becomes very large due to a unique resonance. The two-photon rate becomes comparable with one-photon transition rates, even for low radiation field intensities. Applications of this phenomenon are discussed.

Drastic spectroscopic perturbations due to dynamics of linearly inactive (sequence forming) and linearly active (progression forming) Jahn–Teller modes

We predict drastic changes of splittings in the v–v sequence of a linearly J–T inactive mode along a progression of a linearly J–T active mode. This phenomenon results from the mutual dynamics of the two modes. Preliminary experimental evidence is in accord with these predictions. Previously observed variations in unique splittings due to vibrational perturbations along J–T progressions resulting from asymmetric deuterations can also be explained using the theoretical analysis developed here.

Engineering reverse saturable absorbers for desired wavelengths

Abstract A variety of applications exist for reverse saturable absorbers (RSAs) in laser science (RSAs are substances whose excited-state absorption cross section is larger than their ground-state absorption cross section at a given wavelength and possess a number of other properties). We propose an approach to designing RSAs at a desired wavelength by construction of dimers of dye molecules which absorb near the wavelength of interest. The dimer ground-state absorption is to a state in which the excitation is spread over both monomeric units and the excited-state absorption commences from this state to the doubly excited electronic state in which both monomeric units are excited.

Jahn-Teller-like progressions in the absence and presence of Jahn-Teller distortions

Abstract Prominent progressions of non-totally symmetric modes may ensue in non-linear molecules in electronic transitions involving degenerate states in the absence of Jahn-Teller distortions. In the presence of Jahn-Teller interactions prominent non-Jahn-Teller progressions may result.

Direct observation of the splitting of the vibrationless level in a Jahn-Teller active state in deuterobenzenes of D2h symmetry

Abstract We have succeeded to record the splitting of the 00 band in the δ = 0.3 Rydberg series of benzene converning to the second ionization limit at 11.49 eV upon reducing the vibrational symmetry to D 2h . This measurement is the first direct observation of a splitting of a vibrationless level of a Jahn-Teller active state caused by a vibrational perturbation. This observation leads one inevitably to ascribe the second ionization process in benzene to a removal of an electron from the 3 e 2 g (σ) orbital.

The assignment of the second ionization process in benzene

Abstract New features were observed by us in the Rydberg series of benzene converging to the second ionization limit. A new interpretation of the vibronic structure and vibronic intensity pattern is presented. An interesting vibronic coupling scheme in which four very nearly degenerate electronic states participate is considered. On the basis of our new experimental observations and the new theoretical analysis presented in this paper we are able to assign the second ionization process in benzene as taking place from the 3e 2g -σ orbital. It is shown that previous vibronic analyses have not given a proper and full description of the problem.

Strongly enhanced infrared vibrational transitions in electronically degenerate states

Various types of strongly enhanced infrared vibrational transitions, with transition moments comparable to those characteristic of allowed electronic transitions in visible or UV spectra, may ensue in spatially degenerate electronic states of nonlinear molecules. The intimate correlation between the intensity of the enhanced infrared vibrational transitions and the anomalous pure rotational microwave spectra and the Stark effect is elucidated. The rules of Child and Longuet‐Higgins with regard to infrared activity and infrared intensity enhancements in electronically degenerate states require revision and extension when properly considering the ramifications of the Jahn–Teller effect and invoking the action of mixed quadratic nuclear potential terms. Significantly, the enhancements of IR transition moments should allow the observation of symmetrical free radicals and molecular ions whose laboratory concentrations are often below the level of detectability for ‘‘normal’’ IR transition strengths. In additio...