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Infrared double resonance spectroscopy has been used to study state-resolved rotational and vibrational energy transfer in vibrationally excited SiH4. Completely specified rotational levels (v,J,Cn) are populated by CO2 laser radiation. Subsequent energy transfer is followed by diode laser transient absorption. The total relaxation efficiencies of the initially populated levels for self-collisions and collisions with Ar and CH4 follow the ordering σ(F2)>σ(A2)>σ(E) and are slightly larger than the Lennard-Jones cross sections. State-to-state rotational energy transfer in the ν4 vibration of SiH4 is extremely state specific. In addition to a differentiation between the A, E, and F symmetry levels, there is a selectivity with respect to the fine-structure levels within each rotational state. A preference for transfer to other levels of the same Coriolis sublevel of ν4 was found. This can be phrased as a Δ(J−R)=0 propensity rule. Principal pathways, only one per J per symmetry, are identified. Within each rotational level, the principal-pathway final states are closely spaced; this effect is related to the clustering of the rovibrational levels of the dyad. Large changes in J are possible in a single collision between silane molecules. A kinetic master equation has been used to model energy flow among rotational levels in silane, from which state-to-state energy transfer parameters could be extracted. Collision-assisted absorption of two CO2 photons into the triad has also been detected. A simple modification of the kinetic analysis allows us to obtain an estimate for the relaxation rate out of the triad levels. These laser pumping and relaxation processes determine the efficiency with which high vibrational levels of silane may be populated by infrared multiple photon excitation.

An infrared double resonance laser spectroscopic technique is used to study state-resolved rotational and vibrational energy transfer in the isotopically substituted methane molecule,13CD4 . Molecules are prepared in a selected rovibrational state by CO2 laser pumping, with the quantum numbers v, J, and Cn completely specified. Measurements of both the total rate of depopulation by collisions, and the rates of transfer into specific final rovibrational states (v′, J′, Cn′ ) are carried out using time-resolved tunable diode laser absorption spectroscopy. The depopulation rates due to collisions between methane and the rare gases are on the order of the Lennard-Jones collision frequencies. Self-relaxation is slightly more efficient than the Lennard-Jones estimate. The rather small relaxation rates are characteristic of a short-range potential, or ‘‘strong-collision’’ regime, expected for a molecule without a dipole moment. The state-to-state energy transfer measurements reveal a dramatic selectivity of rotational energy transfer pathways with respect to the fine-structure rotational states Cn . Relaxation occurs through a surprisingly small subset of the energetically accessible pathways. Also suggested is a preference for population transfer to occur within the initial vibrational angular momentum sublevel of the ν4 (F2 ) vibrational state, which has three sublevels in consequence of Coriolis interaction. This preference can be formulated as a propensity for Δ(R−J)=0 transitions. We deduce that large changes of J(ΔJ∼5) can occur in single collisions between methane molecules, based on a simple kinetic model of the data. This is also characteristic of short-range collisions in which it is likely that no single multipolar interaction dominates. Collisional relaxation between the ν4 and ν2 bending vibrations proceeds more slowly than rotational relaxation, but as fast as transfer among the closely grouped stretching and bend-overtone levels, measured previously in CH4 . No evidence for rotationally specific V–V transfer is found. We discuss an exhaustive spectroscopic analysis of 13CD4 that provides unambiguous spectral assignments for use in detecting vibrationally excited molecules (v4 =1) in specific rotational states.

Vibration–vibration (V–V) and vibration–translation (V–T) energy transfer efficiencies have been measured for the v4=1 mode of silane in collisions with He, Ar, Kr, H2, CH4, and itself, using the time-resolved infrared double-resonance technique. The V–V cross sections are approximately one-third to one-half of the Lennard-Jones cross sections, and show a variation with the nuclear–spin symmetry state (A, E, or F) of the molecule. The ν4 V–T deactivation efficiencies are in the range 0.0001–0.002, with the polyatomic molecules being about an order of magnitude more efficient than the noble-gas atoms. This can be quantitatively interpreted by the vibration–rotation (V–R) resonance transfer model of Poulsen et al. [J. Chem. Phys. 58, 3381 (1973)]. A simple breathing-sphere model does not, however, provide a good representation of the V–T collision efficiencies for rare gas–silane collisions.

Time-resolved infrared double resonance experiments have been carried out on silane. A pulsed CO2 laser is used to pump dyad←ground state transitions, and triad←dyad transitions are probed with a tunable diode laser. Two-photon (triad←ground state) signals are observed with the CO2 10P(20), 10P(22), and 10P(28) pump lines. Rotational relaxation rates have been measured for E, F2, and A2 symmetry components of the v4=1, J=13 level of silane in collisions with silane, argon, and methane. The relaxation efficiencies follow the order σrot (F2)>σrot (A2)>σrot (E), which parallels the behavior of pressure-broadening coefficients for infrared absorption lines of methane.

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The magnetic field dependence of the water proton nuclear magnetic relaxation induced by a nitroxide radical both free and covalently bound to serum albumin has been measured from 0.23 mT to 0.7 T. The field dependence of the water proton 1/T1 shows only a minor dependence on the degree of radical immobilization, though the electron spin resonance (ESR) spectrum is very sensitive to the correlation time of the radical. The results and theory are described in more detail elsewhere (Journal of Chemical Physics, in press). The observed relaxation behavior arises because the long electron relaxation time of the nitroxide forces the correlation time for the magnetic interaction driving the water proton relaxation to become the correlation time for the relative translational motion of the water and the nitroxide. The relative insensitivity of the water proton relaxation to nitroxide immobilization should not change if the nitroxide is used as a contrast reagent for magnetic imaging in vivo.

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