Robert L. Whetten

Higher excited states of benzene: Polarized ultraviolet two-photon absorption spectroscopy

The Rydberg spectrum of benzene is explored by means of linear and circular polarized ultraviolet two‐photon absorption spectroscopy of the jet‐cooled molecule. A strong gerade Rydberg series previously designated nRg and all its associated vibronic structure seen in linear polarization is shown to have symmetry A1g, but analysis of spectra obtained under circular polarization demonstrates that E2g origins lie just to lower energies. These combined facts give an unambiguous assignment of this series as nd1 (E2g, A1g, A2g). Four new Rydberg series, each having vibrational structure nearly identical to each other and to the ground state ion, are also identified and have E1g or E2g symmetry. The five Rydberg series converging to the first ionization potential have quantum defects 0.77, 0.06, 0.05, 0.02, and −0.11. Bandwidths indicative of ultrafast radiationless decay processes are observed for low‐n vibronic states, but higher states are sharp. In addition, an Eg band system is observed at 68u2009980 cm−1 (69u20093...

Photodissociation dynamics of Fe(CO)5: Excited state lifetimes and energy disposal

By examining the competition between fragmentation and photoionization of selectively photoexcited Fe(CO)5 in a pulsed jet, we have been able to estimate wavelength dependent upper limits for excited state lifetimes. These range from 0.6 to 1.0 ps over the region from 290 to 310 nm. The nascent CO produced by this dissociation and subsequent multiple fragmentation is detected with rovibronic specificity by two‐photon resonant four‐photon ionization. Results on dynamics and lifetimes favor a dissociation model involving an unbound lowest ligand field state densely mixed with higher‐lying ligand‐field and charge transfer states. We also report the observation of highly efficient ion–molecule fragmentation reactions of Fe(II) with cold Fe(CO)5, in which fragmentation branching ratios depend critically on the Fe(II) states produced by preceding resonance‐enhanced two‐photon ionization of free Fe atoms.

Ultraviolet two‐photon spectroscopy of benzene: A new gerade Rydberg series and evidence for the 1 1E2g valence state

In the first application of continuously tunable ultraviolet two‐photon absorption spectroscopy to a study of the higher excited states of a polyatomic molecule, we have obtained the two‐photon absorption spectrum of jet‐cooled benzene in the energy region from 55u2009000 to 75u2009000 cm−1. The strongest features of this spectrum, seen as two‐photon resonance structure in three‐photon ionization, are assigned to members of a new Rydberg series. Vibronic elements of these Rydberg states show evidence of dynamical effects associated with a Jahn–Teller instability in the 2E1g ionic core toward which the series converges. Notably, the magnitude of vibronic coupling terms in these states appears quite comparable to that present in C6H6+ and the sym‐halobenzene cation ground states, while differing significantly from theoretical calculations predicting ν8 (ring stretch) as the major distorting mode. In addition, we observe two other new band systems: The first, with an origin at 60u2009800 cm−1, has vibronic structure, is...

Higher excited states of benzene: Symmetry assignments of six gerade Rydberg series by four-photon absorption spectroscopy

We report the observation of six gerade Rydberg series in benzene converging to the first ionization potential, seen in the region from 14u2009500–18u2009500 cm−1 as four‐photon resonances in the five‐photon ionization spectrum of the jet‐cooled molecule. Polarization ratios measured for each transition in two‐ as well as four‐photon resonance aid in determining excited state symmetries. We assign a Rydberg series with a quantum defect of 0.76 to higher members of the 3s Rydberg state first seen by Johnson in the near UV two‐photon resonant three‐photon ionization spectrum of benzene. The observed quantum defect, polarization ratios, and correlation with synchrotron absorption data all strongly support this assignment. Another series, with a quantum defect of 0.24, exhibits a polarization ratio of 4.5±0.5 signifying pure rank four activity. This is the first report of a purely rank four transition in a molecule. We assign this as an excitation to a d2 orbital which when converging to the first ionization potentia...

The dynamic Jahn–Teller effect in sym‐triazine: Nonadiabatic wave functions and hindered fluxionality

The complex vibronic structure resolved in the two‐photon absorption spectrum of the 3s 1E’ Rydberg state of sym‐triazine is quantitatively assigned in terms of a simple Jahn–Teller two‐state electronic Hamiltonian coupled to second order by a single active mode, ν6. For Jahn–Teller linear and quadratic terms, k=2.14, and g=0.046 (in units of the zeroth‐order frequency), eigenvalues of this Hamiltonian fit positions and splittings of more than 25 measured bands to within an average deviation of 0.5%. Eigenvectors show evidence of strong mixings of adiabatic states and of linear Jahn–Teller wave functions by quadratic (localization) effects, both of which are confirmed by quantitative agreement between measured and calculated band intensities. Adiabatic potential energy surfaces are calculated, and exact nonadiabatic quantum mechanical results are compared with various levels of approximation. This comparison shows that the simple model of an adiabatic free rotor/radial oscillator serves well to qualitativ...

Strong vibronic coupling in molecular Rydberg states

We consider the vibronic properties of molecular Rydberg states in the strong coupling (degenerate) limit of the ion core, and show how the observed optical Rydberg spectrum for such a molecule is closely related to the Franck–Condon spectrum of its Jahn–Teller ion core, regardless of the symmetry labels attached to the Rydberg electronic species. These relationships permit the resolution of a quarter‐century old paradox in the Rydberg spectrum of benzene and give rise to easily verifiable predictions concerning benzene’s vibronic spectrum, which are applied to a recently discovered new Rydberg series. We further suggest that observed deviations from the idealized Rydberg case discussed here will reveal additional details about the coupling of Rydberg electronic and core‐vibronic motion.

Vibronic structure of nonadiabatic and fluxional states: Two‐photon absorption spectroscopy of jet isolated 3s 1E′ sym‐triazine

The nature of nonadiabatic effects in the Rydberg and ground ionic states of jet‐cooled sym‐triazine is investigated by means of ultraviolet two‐photon absorption spectroscopy. A highly resolved band system is observed in the region from 55u2009000 to 60u2009000 cm−1, which can be associated with excitation from an e′ lone pair to the 3s Rydberg orbital. A complete assignment of the low‐energy bands of this system, as determined by isotope effects and comparison to model calculations, reveals a clear example of the dynamical Jahn–Teller effect for the case where only one mode, the ν6 ring distortion, is significantly active. On the basis of simple limiting models we derive approximate vibronic coupling parameters and present a quantitative description of vibronic motion in terms of adiabatic molecular coordinates. We conclude that sym‐triazine, in its ground state one of the most rigid of medium‐sized polyatomics, becomes strikingly fluxional in its Rydberg and ground ionic states, as the dynamic Jahn–Teller effe...

High rydberg states of jet-cooled toluene observed by ultraviolet two-photon absorption spectroscopy: Ultrafast radiationless decay and pseudo-Jahn-Teller effects

Abstract Three new Rydberg series converging to the first ionization potential of toluene are identified by the method of ultraviolet two-photon absorption in a supersonic jet. No Rydberg series converging to the near-degenerate second ionization potential are found, a result analogous to that known for the valence systems of benzenoid molecules. The vibronic spectrum of the toluene cation is discussed in light of this result, and the failure to observe a distinct splitting of the lowest two states is attributed to a complete pseudo-Jahn-Teller mixing of the vibronic states. In addition, the possibility of systematic perturbations between Rydberg series converging to vibrationally excited states of the ion and purely electronic series is examined in light of quantum defect variations. A striking feature of toluenes Rydberg spectrum is the dramatic sharpening of individual bands with increasing principal quantum number. This sharpening is associated with a decrease in radiationless transition rate, a proposition which is supported by agreement between present bandwidth measurements and the direct femtosecond decay measurement of Wiesenfeld and Greene. Evidence favoring a spiraling decay mechanism is summarized.

Observation of multimode vibronic interference effects in 2E1g benzene

This paper presents the first experimental demonstration of multimode vibronic interference effects arising from the interaction of topologically inequivalent vibrational modes with a two‐level electronic degeneracy. The system is that of the E1g molecular ion core of benzene Rydberg states. The interacting modes are the Jahn–Teller active e2g, ring bend, ν6, and the Renner, out‐of‐plane e2u ring bend, ν16. The nature of the interaction is seen as a coupling of the angular momenta of the two modes individually to the electronic angular momentum, producing new quantization conditions, within which interference interactions can be approximated perturbatively. Such a treatment explains qualitatively all splitting patterns observed in experimental spectra. Small quantitative deviations point the way to higher order couplings.

Direct observation of nonlinear Jahn-Teller effects in the 1 1A1g→3s 1Eg two-photon spectrum of cyclohexane

The ultraviolet two‐photon absorption spectrum of the lowest excited singlet state of jet‐cooled cyclohexane reveals a highly perturbed system of very sharp bands in the 55u2009000–58u2009000 cm−1 region, which we assign as transitions to vibronic elements of the 3su20091Eg Rydberg state. On the basis of polarization, isotope, and variable‐temperature data and by use of vibronic coupling calculations, we construct a detailed assignment of the vibronic level structure which demonstrates that the proliferation of low‐energy states is a direct result of the introduction of an entirely new type of low‐frequency motion (150 cm−1) in the excited state, that of the Jahn–Teller vibronic pseudorotation. We observe moderate to high activity in five of the eight possible Jahn–Teller modes, and estimate a total Jahn–Teller stabilization energy of 2550 cm−1. Importantly, we find large splittings (15% of the zeroth‐order frequency) of states which would remain degenerate given only linear Jahn–Teller activity (free pseudorotation)...