Eric Weitz

Calculations of the surface temperature rise and desorption temperature in laser-induced thermal desorption

The laser‐induced thermal desorption (LITD) process is discussed with reference to measurements of the translational energies (temperatures) of desorbed molecules. We present simple expressions which predict both the surface temperature rise for a laser‐heated surface and the desorption temperature in LITD. We show that these approximations are excellent descriptions of the laser heating and LITD processes. These expressions are convenient alternatives to numerical solutions of the heat conduction and desorption rate equations using the actual shape of a laser pulse and specific combinations of kinetic parameters.

Detection of Transient Organometallic Species by Fast Time-Resolved IR Spectroscopy

Publisher Summary This chapter discusses the role of matrix isolation in characterizing transition-metal fragments and then considers what conventional flash photolysis with uv-vis detection has revealed about the reactivity of these fragments. It is the timescale of these reactions that dictates the speed of the infrared (IR) spectroscopy required to detect the intermediates. The principles of these new IR techniques are explained and the apparatus involved is described. The chapter presents a self-contained summary of the organometallic chemistry that has already been unraveled by time-resolved IR spectroscopy. The basic principles of matrix isolation are relatively well known, and its application to organometallic chemistry has been recently reviewed. Many of the species generated in low-temperature matrices are coordinatively saturated species. Unlike the unsaturated transition-metal fragments, these species may have significant activation barriers for reaction or decomposition and can be stabilized by merely lowering the temperature. IR kinetic spectroscopy involves uv flash generation of transients and monitoring of transients at a finite number of IR wavelengths. There have been three primary motives behind the study of metal–carbonyl photochemistry in the gas phase: to discover the shapes of metal–carbonyl fragments in the absence of perturbing solvents or matrices, to probe the effect of uv photolysis wavelength on product distribution, and to measure the reaction kinetics of carbonyl fragments.

Overtone spectroscopy of propyne and propyne‐d1

The vibrational overtone spectra of the acetylenic and methyl C–H stretches of propyne were obtained for the v=1 to v=6 and v=1 to v=7 levels, respectively. Propyne‐dl was also studied and the methyl C–H stretching overtones were measured from v=1 to the v=7 level. The C–D stretch was observed only in the fundamental and first overtone regions. Lower level overtones were obtained by standard infrared techniques, while higher absorptions (>12u2009000 cm−1) were obtained by intracavity dye laser photoacoustic spectroscopy. The C–H stretches in both molecules were analyzed in terms of the local‐mode model, and harmonic frequencies (ωi) and anharmonicities (Xii) were calculated. In propyne these values were (acetylenic C–H stretch) ω1=3384±5 cm−1 and X11=−50±1 cm−1 and (methyl C–H stretch) ωm =3037±5 cm−1 and Xmm =−65±2 cm−1. In propyne‐dl the methyl C–H stretch parameters were ωm =3034±5 cm−1 and Xmm =−64±2 cm−1. For propyne, a hot band (ν9→ν9+vν1) accompanying the acetylenic C–H stretch was observed for v=1–6 a...

Vibrational relaxation of HCl in HCl/liquid xenon mixtures

Abstract The vibrational relaxation of HCl in liquid xenon at −68°C has been studied. Vibrationally excited HCl is created following UV photolysis of HCl. The rate of relaxation of HCl as a function of HCl mole fraction in liquid xenon is the same as the corresponding rate for gas phase HCl at −68°C. Similarly the rate of relaxation of HCl in HClue5f8Xe collisions is in good agreement with the corresponding gas phase rate. These observations imply that the isolated binary collision model is valid for this system.

Associative substitution reactions in 17 electron organometallic radicals: direct observation of the reaction of [(C5H5)Fe(CO)2]· with P(OMe)3 using very fast time-resolved i.r. spectroscopy

Time-resolved i.r. spectroscopy is used to monitor directly the formation of [(C5H5)Fe(CO)P(OMe)3]· in the reaction of P(OMe)3 with [(C5H5)Fe(CO)2]·, generated by 351 nm photolysis of [{(C5H5)Fe(CO)2}2] in n-heptane solution at room temperature; the reaction is an associative process with a bimolecular rate constant of 8.9 ± 2.0 × 108 dm3 mol–1 s–1.

Relaxation of vibrationally excited HCl in solid Xe

Abstract Rates for vibrational relaxation of HCl(ν = 1.2) in solid xenon at 40 and 146 K are reported and are compared to the rate of relaxation of HCl(ν = 1) in liquid xenon near the freezing point. Upon freezing, the rate of relaxation of HCl(ν = 1) is found to decrease significantly and emission from HCl(ν = 2), absent in the liquid phase, is detected. Both of these effects are attributed to a significant decrease in mobility of HCl molecules in the solid phase as compared to the liquid phase. At both 40 and 146 K, the ratio of relaxation rates for HCl(ν = 2) to HCl(ν = 1) is found to deviate significantly from the harmonic oscillator prediction of 2:1. The rate of relaxation for HCl(ν = 1) by xenon is found to be similar in both liquid solution at 200 K and in the solid at 146 K.

Vibrational energy transfer in solutions: From diffusive to impulsive binary collisions

The effect of diffusion on energy transfer from excited donor to acceptor molecules in liquid solutions is studied with particular attention focused on vibrational energy transfer between solute molecules in dilute solutions. Such processes are often discussed in the independent binary collision (IBC) framework and diffusion effects are assumed to be negligible. We introduce the concept of diffusive collisions (encounters between acceptor and donor molecule within an effective energy transfer range) and investigate the conditions under which the cross section for the energy transfer process may be affected by the cross section for the diffusive collision as opposed to the more common fast diffusion limit where the energy transfer is dominated by direct binary collisions. We conclude that while in most common situations vibrational energy transfer is indeed dominated by binary collision events, pronounced diffusion effects should exist at moderately high pressures. Explicit estimates are provided for the H...

The vibrational relaxation of DCl(v=1) in liquid xenon and HCl(v=1) in liquid krypton

The relaxation rate constants for DCl(v=1) in liquid xenon at 211 K and HCl(v=1) in liquid krypton at 190 K are reported. Both the DCl–DCl and HCl–HCl rate constants are similar to the corresponding gas phase rate constants at the same temperature, and the relaxation appears amenable to an isolated binary collision (IBC) description. As in the gas phase, the rate constant for DCl–DCl deactivation in liquid xenon is found to be smaller than that for HCl–HCl relaxation in liquid xenon, despite the smaller energy defect for the DCl relaxation process. This suggests that rotational degrees of freedom are also involved in the liquid phase vibrational relaxation of these molecules. The observed relaxation rates for DCl(v=1) by liquid xenon and HCl(v=1) by liquid krypton are found to be within an order of magnitude of values estimated from various forms of the IBC model. More careful comparisons await values of the appropriate gas phase relaxation rate constants near 200 K. Finally, certain features of the mecha...

Relaxation of HCl in liquid xenon solution by vibration–vibration energy transfer

The deactivation of vibrationally excited HCl produced via excimer laser photolysis of chloroethylenes has been studied in both the gas phase and in liquid xenon solution. The rate of deactivation of HCl* by the chlorethylene parent is significantly faster in liquid xenon than in the gas phase. This occurs due to enhancement of the vibration–vibration energy transfer probability in solution. The nature of this enhancement is discussed.

Infrared multiphoton isomerization and fragmentation reactions of organic molecules

The tunability of the CO2 molecular gas laser permits selective multiphoton infrared excitation of one isomer in a mixture of isomers. This capability can be exploited to drive isomerization reactions in a contra— thermodynamic direction. Examples of such reactions are the trans + cis isomerization of alkenes and the electrocyclic isomerization of butadienes to cyclobutenes. It is also possible to achieve some selectivity in consecutive isomerization reactions (A + B + C) and in competing reactions (A + B ÷ C) via multiphoton infrared excitation. For example, consecutive reactions can be stopped at the intermediate product B in cases where the activation energy for B + C is lower than that for A + B. The product ratio B/C in competing reactions is, in some cases, dependent upon the average vibrational temperature of the reactant A and hence can be altered by changing laser fluence or collisional frequency. In addition, pulsed infrared lasers can be used to create high concentrations of vibrationally excited fragmentation products. We are currently seeking to correlate product vibrational energy distributions with reactant structure and decomposition mechanism.