Alan Furlan

Cars spectra of the HCl dimer in supersonic jets

Abstract The ν 2 + and ν 2 − HCl stretching bands of (HCl) 2 have been detected at 0.3 cm −1 resolution by coherent anti-Stoke spectroscopy (CARS) in supersonic expansions of HCl seeded in rare gases. By combination of the transition frequencies with complementary IR data, the tunneling splittings ν 0 T and ν 2 T are determined.

The photodissociation dynamics of OClO between 306 and 370 nm: Fragment translational energy release and recoil anisotropy

The photodissociation OClO(A 2A2)→ClO(X 2Π)+O(3P) was studied at wavelengths between 306 and 370 nm using photofragment translational energy spectroscopy. The flight time distributions and anisotropies of the recoiling fragments were measured with the photolysis wavelength tuned to 10 maxima of the structured absorption spectrum, corresponding to a vibronic excitation of the parent molecule with 9–18 quanta in the symmetric stretching coordinate on the A 2A2 surface. The translational energy distributions show that the ClO fragments are created in highly inverted vibrational state distributions which become extremely broad [v(Cl–O)∼1–15] with increasing excitation energy. The large fraction of vibrationally hot ClO fragments produced–particularly at λ<325 nm–could enhance various thermodynamically unfavorable atmospheric reactions in connection with ozone depletion. The main mechanistic features of the dissociation process, which account for the almost constant average translational energy and linearly i...

THE PHOTODISSOCIATION OF CARBONYL CYANIDE CO(CN)2 AT 193 NM STUDIED BY PHOTOFRAGMENT TRANSLATIONAL ENERGY SPECTROSCOPY

The photodissociation of carbonyl cyanide CO(CN)2 at 193 nm was investigated by photofragment translational energy spectroscopy. For all the fragments created (CO, CN, OCCN, NCCN), the kinetic energy distributions were measured and two decay channels identified. The radical decay, CO(CN)2+hν→OCCN+CN, dominates with a yield of 94%±2% and shows the available energy mainly (82%) channeled into the internal degrees of freedom of the fragments. A fraction of 18%±6% of the nascent OCCN radicals has sufficient energy to spontaneously decay to CO+CN involving a barrier ⩽160 kJ/mol. With a yield of 6%±2% the molecular decay produces the fragments CO+NCCN. These fragments acquire a high available energy owing to the formation of the new C–C bond in NCCN. An average fraction of 70% is partitioned into internal fragment energy. Even the fastest fragments are still internally hot, indicating that with the high barrier expected, a substantial exit channel interaction is operative. The isotropic recoil distribution foun...

The Jahn–Teller effect in triptycene

The irregular vibronic structure resolved in the S1←S0 resonant two‐photon ionization (R2PI) spectrum of supersonically cooled triptycene (9,10‐dihydro‐9,10[1’2’]benzenoanthracene) is assigned in terms of a single‐mode E’⊗e’ Jahn–Teller vibronic Hamiltonian for the excited state, with linear and quadratic coupling terms. The Jahn–Teller active vibrational mode is a benzene wagging framework mode. To fit to the observed vibronic levels yields a very low frequency νe’ =47.83 cm−1 and linear and quadratic terms are k=1.65 and g=0.426. This fit accounts for ≊98% of the observed absorption band intensities over the observable range 0–350 cm−1. The quadratic term is unusually large, leading to localization of the lowest vibronic levels in the three symmetry‐equivalent minima. Emission spectra from 13 vibronic levels in the excited E’ state show extended vibrational progressions with up to 25 members in the analogous e’ ground state vibration, which is highly harmonic in the electronic ground state. The Franck–C...

Isomer- and “phase”-selective spectroscopy of van der Waals solvent clusters

At the atomic size level, descriptions of rearrangement of structures and shapes of molecules are couched in chemical terminology (isomerization, racemization, rearrangement, etc.). In bulk solids, structural changes are described on a collective scale as order-disorder transformations and phase transitions (melting). To describe structural changes in atomic and molecular clusters we employ elements of both pictures:(a) local bonding rearrangements in molecules, and(b) collective permutational and/or translational rearrangements in the bulk. Various examples are given of isomerism and collective structural transformations in clusters.

Coupling of a Jahn–Teller pseudorotation with a hindered internal rotation in an isolated molecule: 9-hydroxytriptycene

The irregular vibronic structure in the S1←S0 resonant two-photon ionization (R2PI) spectrum of supersonically cooled triptycene is a result of a classic E⊗e Jahn–Teller effect [A. Furlan et al., J. Chem. Phys. 96, 7306 (1992)]. This is well characterized and can be used as an effective probe of intramolecular perturbations. Here we examine the S1←S0 R2PI spectrum of 9-hydroxytriptycene and the fluorescence from various excited state vibronic levels. In this system the pseudorotation of the Jahn–Teller vibration is strongly coupled to the torsional motion of the bridgehead hydroxy group. This torsional motion results in a tunneling splitting in both the ground and excited states. The population of the upper level in the ground electronic state results in additional vibronic transitions becoming symmetry allowed in the R2PI spectrum that are forbidden in the bare triptycene molecule. The assignment of the R2PI and fluorescence spectra allows the potential energy surfaces of these vibrational modes to be ac...

A Trimer Vibronic Coupling Model for Triptycene - the Jahn-Teller and Barnett Effects

The first singlet electronic excited state of triptycene, as measured by resonant two‐photon ionization in a supersonically cooled beam, has been found to be a textbook example of the E’⊗e’ Jahn–Teller effect. Here it is shown that this E’⊗e’ vibronic coupling can be profitably viewed as a subset of a (A’1⊕E’)⊗(a’2⊕e’) vibronic coupling scheme which results from a simple trimer model. The enlarged coupling scheme has a simple physical interpretation where the wagging coordinates of the benzene subunits are strongly coupled to their excimer formation. The previously obtained parameters, in which there is a large reduction between the ground and excited electronic state frequencies of the lowest frequency e’ mode as well as an unusually large second‐order vibronic coupling constant, are shown to arise naturally from a trimer viewpoint. Features of the spectra have been found which are attributed to the involvement of an a’2 vibration which couples through nonzero momentum rather than coordinate matrix eleme...

Intermolecular perturbation of a Jahn–Teller system: The triptycene⋅Nen (n=1–3) van der Waals clusters

The effect of rare gas complexation on the electronically excited S1(E’) state of triptycene (T), which is Jahn–Teller distorted, was investigated by two‐color resonant two‐photon ionization (2C‐R2PI) spectroscopy of the supersonically cooled van der Waals complexes triptycene⋅Nen, n=1–3. These complexes afford unique possibilities to study the effects of weak intermolecular interactions on the intramolecular Jahn–Teller coupling. Since the atoms are adsorbed at high‐symmetry positions, the system symmetry is lowered from D3h(n=0) to C2v for n=1 and 2, but reverts to D3h for n=3. A Jahn–Teller (A1⊕E)⊗e coupling model including a uniaxial external strain component was applied successfully to calculate the S1 state levels and S1←S0 electronic spectra of all three complexes. The spectrum of T⋅Ne3 was fully interpreted without inclusion of strain, implying a highly symmetric D3h structure in which each of the three V‐shaped compartments of triptycene is occupied by a single Ne atom. In contrast, the vibronic ...

The Jahn–Teller effect in 9‐fluorotriptycene

The vibronic structure in the S1(E)←S0(A1) resonant two‐photon ionization (R2PI) spectrum of supersonically cooled 9‐fluorotriptycene is assigned using three different Jahn–Teller (JT) model Hamiltonians for the excited 1E state—E⊗e, (A⊕E)⊗e, and (A⊕E)⊗(a2+e). The basic E⊗e interpretation is satisfactory. However, the fitted vibronic band frequencies and intensities are improved by including coupling to a second excited state 1A1 in an exciton model. Some further observed absorption bands are only assignable by invoking a molecular Barnett effect (momentum coupling to an a2 vibration). The measured fluorescence emission spectra from different S1 vibronic levels are quantitatively reproduced within all three coupling schemes by the parameters fitted to the R2PI spectrum. Results are compared to previous calculations on unsubstituted triptycene. The JT stabilization energy is decreased by ∼10% upon fluoro bridgehead substitution, which is rationalized by the electron‐withdrawing effect of the F atom. For th...

The Jahn-Teller and Barnett effects

Abstract In the usual formulation of the Jahn-Teller effect a simplification is made in going from the adiabatic to the crude adiabatic approximation in which the electronic parts of the vibronic wavefunction are assumed independent of the nuclear coordinates. This then neglects momentum coupling in the vibronic coupling matrix. The momentum coupling has been termed the molecular Barnett effect when the active vibration transforms as the irreducible representation of a rotation in the molecular point group. Experimental evidence for the molecular Barnett effect has recently been found. In this paper the various point groups in which momentum and Barnett coupling can occur are investigated. A vibration capable of momentum coupling is contained in the asymmetric direct product of the degenerate electronic state and, as with the Jahn-Teller effect, is possible in the orbitally degenerate electronic states of molecules of all non-linear point groups. A static distortion along such a coordinate will lift the electronic degeneracy. Unlike the Jahn-Teller effect, however, in some point groups a minimum complexity of the molecule is required before such coupling can occur. In particular it will be absent in the degenerate electronic states of such simple molecules of the form X 3 (D 3h ); XY 3 (C 3v , D 3h ); XY 4 (T d ); and XY 6 (O h ).