Colin M. Western

Dissociation dynamics of HCN(DCN) following photoexcitation at 121.6 nm

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of HCN(DCN) at 121.6 nm. Analyses of the H(D) atom time‐of‐flight spectra reveal the partner CN fragment to be formed predominantly in its A 2Π excited electronic state. Branching into the H/D+CN(B 2∑+) product channel accounts for a few percent of the total fragment yield, but we discern no evidence for any contribution from the product channel yielding H/D atoms in conjunction with ground state CN(X 2∑+) fragments. The majority of the CN (A) fragments are formed in their v’=0 level but with a markedly bimodal rotational state population distribution. This bimodality has been rationalized in the light of the available information regarding the form of the potential energy surface of the excited 1Π state of HCN(DCN) populated following photoexcitation at 121.6 nm.

Translational spectroscopy of H(D) atom fragments arising from the photodissociation of H2S(D2S): a redetermination of D00(S–H)

The technique of H/D atom photofragment translational spectroscopy has been used to further investigate the collision-free photodissociation of H2S and D2S molecules both in the near ultraviolet (at 218.2 and 221.6 nm) and in the vacuum ultraviolet (at 121.6 nm). Measurements of the H/D atom photofragment angular distributions confirms that the near UV dissociation occurs promptly, following a perpendicular photo-excitation. More than 99% of the resulting SH/SD fragments are formed in their ground vibronic level, with a ca. 3 : 2 preference in favour of the lower (2Π3/2) spin–orbit component. Product rotation accounts for ca. 1% of the available energy in the case of H2S photolysis at these near UV wavelengths (ca. 2% in the case of D2S dissociation). The groundstate SH/SD photofragments can also be photolysed at these near UV excitation wavelengths. Simulations of the kinetic energy distribution of the resulting H/D atomic fragments show that the secondary photolysis also involves a perpendicular transition, and that the partner S atoms are formed in all three 3PJ spin–orbit states. The product energy disposal following 121.6 nm photolysis of D2S closely parallels that deduced in an earlier study of H2S photodissociation at this same wavelength (Schnieder et al., J. Chem. Phys., 1990, 92, 7027). The D-atom kinetic energy spectrum shows clear evidence for the formation of rovibrationally excited SD(A 2Σ+) fragments amongst the primary products, and also suggests an important role for the three-body dissociation process leading to D + D + S(1D) atoms.Given D00(HS–H)= 31 480 ± 40 cm–1, the present results provide a refined value for the S–D bond strength in the D2S molecule; D00(DS–D)= 32 030 ± 50 cm–1; for the SH and SD radical bond dissociation energies; D00(S–H)= 29 300 ± 100 cm–1 and D00(S–D)= 29 700 ± 100 cm–1, and an improved expression for the potential-energy function for the A 2Σ+ state of the mercapto radical.

The interpretation of molecular magnetic hyperfine interactions.

Investigations of the hyperfine structure in the excited electronic states of several free radical species have revealed shortcomings in the currently accepted values used for the theoretical interpretation of such interactions. We introduce updated reference atomic values from a combination of experimental observations and ab initio calculations. The latter are at Hartree-Fock and multireference configuration interaction levels of theory and several atomic test cases are discussed. Furthermore, ground and excited electronic state hyperfine coupling constants are calculated using both levels of theory for a range of first- and second-row diatomic hydride and nonhydride radicals. These results, together with a selection of other experimental measurements are then compared with experimental data where available, and the implications of the revised interpretation are discussed.

Multiphoton ionisation spectroscopy of free radical species

We survey the impact of resonance-enhanced multiphoton ionisation methods on our knowledge and understanding of the spectroscopy, the structure and the dynamics of excited electronic states of free radical species. Four radicals, the diatomics NH and SO, and the somewhat more complex methyl and allyl radicals, have been chosen as case studies to illustrate some of the many merits of this spectroscopic technique. We conclude with an anticipation of some of the likely future directions for this area of research.

A desorption mechanism of water following vacuum-ultraviolet irradiation on amorphous solid water at 90 K

Following 157 nm photoexcitation of amorphous solid water and polycrystalline water ice, photodesorbed water molecules (H(2)O and D(2)O), in the ground vibrational state, have been observed using resonance-enhanced multiphoton ionization detection methods. Time-of-flight and rotationally resolved spectra of the photodesorbed water molecules were measured, and the kinetic and internal energy distributions were obtained. The measured energy distributions are in good accord with those predicted by classical molecular dynamics calculations for the kick-out mechanism of a water molecule from the ice surface by a hot hydrogen (deuterium) atom formed by photodissociation of a neighboring water molecule. Desorption of D(2)O following 193 nm photoirradiation of a D(2)O/H(2)S mixed ice was also investigated to provide further direct evidence for the operation of a kick-out mechanism.

The 1E″ state of NH3: the Jahn-Teller effect revealed by infrared-optical double resonance

Several new vibronic levels of the 1E″ state of NH3 have been measured using infrared-optical double resonance and supersonic jet spectroscopy. Rotational analyses of these bands are presented and an analysis of the overall vibronic structure is also given. A small but significant Jahn-Teller distortion is found to be present (less than the zero point motion). Vibronic interactions with nearby electronic states are required to explain the observed pattern of fibronic energy levels.

Ab initio calculations and vibrational energy level fits for the lower singlet potential-energy surfaces of C3.

Ab initio multireference configuration interaction potential energy surfaces are computed for the eight lowest singlet surfaces of C(3). These reveal several important features, including several conical intersections in linear, nonlinear, and equilateral triangle geometries. These intersections are important because, particularly for the excited A (1)Pi(u) state, reasonable ab initio results could only be obtained by including nearby, near degenerate, (1)Sigma(u) (-) and (1)Delta(u) states that cross the A (1)Pi(u) state around 4500 cm(-1) above the equilibrium geometry, and a (1)Pi(g) state whose potential in turn crosses the other states about 2000 cm(-1) further up. These states are probably responsible for the complexity of the shorter wavelength UV absorption spectrum of C(3). The computed potential energy surface for the ground, X (1)Sigma(g) (+), state and for the lowest two excited singlet surfaces (which both correlate with the A (1)Pi(u) state in a collinear geometry) are fitted to analytic functional forms. Vibrational energy levels are calculated for both states, taking account of the Renner-Teller coupling in the excited A (1)Pi(u) state. The potential parameters for both states are then least-squares fitted to experimental data. The ground-state fit covers a range of approximately 8500 cm(-1) above the lowest level, and reproduces 100 observed vibrational levels with an average error of 2.8 cm(-1). The A (1)Pi(u) state surfaces cover a range of 3250 cm(-1) above the zero-point level, and reproduce the 44 observed levels in this range with an average error of 2.8 cm(-1).

The spectroscopy of high Rydberg states of ammonia

This paper extends our knowledge of the higher excited states of the ammonia molecule by presenting detailed measurements of the 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum of both NH3 and ND3 obtained following excitation in the wavelength range 298–242 nm, i.e., at energies up to the first ionization energy. Complementary analyses of the wavelength resolved REMPI spectrum and the accompanying REMPI-photoelectron spectra leads to the identification of ten new Rydberg origins of NH3 (four for ND3) with principal quantum numbers n⩽8 and, in most cases, of the accompanying out-of-plane bending vibrational progression. Symmetry assignments for the various newly identified excited states are offered, based on band contour simulation and/or quantum defect considerations. Dominant amongst these are the Ẽ″ 1A2″ (5sa1′←1a2″) state: ν0=74 118(2) cm−1 [NH3], ν0=74 258(2) cm−1 [ND3], the F″ 1E″ (5pe′←1a2″) state: ν0=76 220(50) cm−1 [NH3], ν0=76 240(50) cm−1 [ND3], the F′ 1A1′ (5pa2″←1a2″) state:...

The A 3Π state of SO

Laser induced fluorescence and 1+1 resonance enhanced multiphoton ionisation spectra of the A 3Π–X 3Σ− transition of SO radicals prepared by an electric discharge in a supersonic jet expansion are presented. Rotational constants are given for A state vibrational levels with v′=0–13, extending to within 190 cm−1 of the A state dissociation limit. The Rydberg–Klein–Rees curve derived from these constants shows significant anharmonicity, even around the equilibrium geometry. In addition, several small local perturbations of the rotational structure are observed. Collision free fluorescence lifetimes are determined for the complete range of vibrational states, and are found to fall smoothly from 29.5 μs for v′=0 to 6.45 μs for v′=12. Combining these data with earlier measurements leads to a better determination of the A–X transition dipole moment over the range 1.4–2.0 A.

HYPERFINE MEASUREMENTS IN THE X AND B ELECTRONIC STATES OF I35,37CL : PROBING THE IONIC CHARACTER OF THE CHEMICAL BOND

The laser induced fluorescence spectra of the B 3Π(0+)←X 1Σ+ electronic transition of I35Cl and I37Cl have been recorded with a linewidth of ∼5 MHz. This resolution allows the hyperfine splittings due to both nuclei to be measured. We report values for eQq0(I) and eQq0(Cl), for both I35Cl and I37Cl, for v’=0–2 and v‘=0–2. This provides new data for both the ground and excited states. Information on the ionic character of the chemical bond of ICl in the X and B electronic states was deduced using a single electronic configuration [linear combination of atomic orbitals/molecular orbital (LCAO/MO)] model. The ionic character (fractional charge transfer) of the X state is −0.24 for v‘=2, 1% less than for v‘=0. Since the π and π* orbitals are completely filled for the X state, this ionic character is due entirely to the σ bonding orbital. For the B electronic state, the total ionic character was estimated from the dipole moment, and the hyperfine data were used to separately determine the ionic character of th...