Robert M. Dickson

Selection rules for nuclear spin modifications in ion-neutral reactions involving H3+

We present experimental evidence for nuclear spin selection rules in chemical reactions that have been theoretically anticipated by Quack [M. Quack, Mol. Phys. 34, 477 (1997)]. The abundance ratio of ortho-H3+ (I=3/2) and para-H3+ (I=1/2), R=[o-H3+]/[p-H3+], has been measured from relative intensities of their infrared spectral lines in hydrogen plasmas using para-H2 and normal-H2 (75% o-H2 and 25% p-H2). The observed clear differences in the value of R between the p-H2 and n-H2 plasmas demonstrate the spin memory of protons even after ion-neutral reactions, and thus the existence of selection rules for spin modifications. Both positive column discharges and hollow cathode discharges have been used to demonstrate the effect. Experiments using pulsed plasmas have been conducted in the hollow cathode to minimize the uncertainty due to long-term conversion between p-H2 and o-H2 and to study the time dependence of the o-H3+ to p-H3+ ratio. The observed R(t) has been analyzed using simultaneous rate equations ...

High-resolution laser spectroscopy of the Qv(0) transitions in solid parahydrogen

Our recent high-resolution laser spectroscopy of theQv(0) (υ=υ←0,J=0←0) transitions in solid parahydrogen is discussed. The systems studied include the fundamental vibrational bands of impurity D2 and HD, the first and second overtones of parahydrogen, and the charge-induced spectrum ofγ-ray irradiated parahydrogen. Additionally, Stark and stimulated Raman-gain spectroscopies are applied to the solid. The linewidths are as sharp as 2 MHz HWHM, which is highly unusual for a solid. Our spectra demonstrate a variety of physical phenomena, particularly theΔk = 0 selection rule, as well asJ = 1/J = 0 pair intermolecular interactions.

Charge induced H2 spectrum in γ‐ray irradiated para‐H2 crystals

A sharp spectral line has been observed in γ‐ray irradiated para‐H2 crystals and assigned to the pure vibrational Q1(0)(v=1←0, J=0←0) transition induced via the Condon effect by charges distributed in the crystal. The remarkable sharpness and stability of the signal is reported and discussed.

Direct measurement of the crystal field splitting of isolated J=1 impurities in solid parahydrogen

The crystal field splitting of single J=1 impurity orthohydrogen molecules in an otherwise pure J=0 parahydrogen crystal has been studied experimentally and theoretically over many years. We have measured this quantity directly from the fine structure of the Q3 (0) (v = 3← 0, J = 0← 0) overtone transition induced by J=1 hydrogen, in the near infrared using a Ti:sapphire laser. From our observed eight pairs of split spectral lines, we have determined the value Δ/k = [E(M=±1) − E(M=0)]/k = 0.0102 ± 0.0003 K including its sign. A straightforward numerical calculation based on ab initio calculations of the H2-H2 potential reproted by Mulder, van der Avoird, and Wormer gives Δ/k = 0.0116 K which is close to the experimental value. This suggests that the static and dynamic effects of phonons on the splitting may have been overestimated in the past.