D. Krajnovich

Infrared vibrational predissociation spectroscopy of water clusters by the crossed laser‐molecular beam technique

Water clusters formed in a molecular beam are predissociated by tunable, pulsed, infrared radiation in the frequency range 2900–3750 cm−1. Absorption spectra of the clusters are obtained by detecting the recoiling fragments off‐axis from the molecular beam as a function of laser frequency using a rotatable mass spectrometer. By carefully adjusting the expansion conditions of the molecular beam and monitoring the largest cluster observable, excessive contamination by clusters larger than the specific one of interest is avoided. It is found that the spectra of clusters containing three or more water molecules absorb over the same frequency range as the liquid. Dynamical information on the predissociation process is obtained from the measured angular and velocity distributions of the fragments. An upper limit to the excited vibrational state lifetime of ∼1 μs is observed for the results reported here. The most probable dissociation process concentrates the available excess energy into the internal motions of...

The photodissociation of nitromethane at 193 nm

The dissociation of nitromethane following the excitation of the π* ← π transition at 193 nm has been investigated by two independent and complementary techniques, product emission spectroscopy and molecular beam photofragment translational energy spectroscopy. The primary process is shown to be cleavage of the C–N bond to yield CH3 and NO2 radicals. The translational energy distribution for this chemical process indicates that there are two distinct mechanisms by which CH3 and NO2 radicals are produced. The dominant mechanism releasing a relatively large fraction of the total available energy to translation probably gives NO2 radicals in a vibrationally excited 2B2 state. When dissociated, other nitroalkanes exhibit the same emission spectrum as CH3NO2, suggesting little transfer of energy from the excited NO2 group to the alkyl group during dissociation for the dominant mechanism. This conclusion is supported by the apparent loss of the slow NO2 product in the molecular beam studies to unimolecular diss...

Competition between atomic and molecular chlorine elimination in the infrared multiphoton dissociation of CF2Cl2

Infrared multiphoton dissociation of CF2Cl2 has been reinvestigated by the crossed laser‐molecular beam technique using a high repetition rate CO2 TEA laser. Both the atomic and molecular chlorine elimination channels were observed: (1) CF2Cl2→CF2Cl+Cl and (2) CF2Cl2→CF2+Cl2. No evidence was found for secondary dissociation of CF2Cl at laser energy fluences up to 8 J/cm2. Center‐of‐mass product translational energy distributions were obtained for both dissociation channels. In agreement with previous work, the products of reaction (1) are found to have a statistical translational energy distribution. The products of reaction (2) are formed with a mean translational energy of 8 kcal/mol, and the distribution peaks rather sharply about this value, indicating a sizeable exit barrier to molecular elimination. The product branching ratio was directly determined. Reaction (2) accounts for roughly 10% of the total dissociation yield in the fluence range 0.3–8 J/cm2. These results provide an additional test of th...

UV photodissociation of C2F5Br, C2F5I, and 1,2‐C2F4BrI

The photodissociation reactions of C2F5Br, C2F5I, and 1,2‐C2F4BrI have been studied at 248 and 193 nm using the crossed laser‐molecular beam technique. Photodissociation of 1,2‐C2F4BrI was also studied at 266 nm. We find that: (i) C2F5Br undergoes C–Br bond fission at 193 nm (but does not absorb at 248 nm); (ii) C2F5I undergoes C–I bond fission at 248 nm (but does not absorb at 193 nm); (iii) 1,2‐C2F4BrI undergoes only C–I bond fission at 248 and 266 nm, but at 193 nm both C–I and C–Br bond fission are observed, with a C–I:C–Br fission ratio of approximately 1.7:1. Center‐of‐mass recoil energy and angular distributions are determined for each of these photodissociation reactions. These results combined with those of other workers, are used to test simple predictions based on molecular orbital theory. The 266 nm data for C2F4BrI provide an approximate value of 19.3±3 kcal/mol for the dissociation energy of C2F4Br to C2F4+Br and also show that all or almost all the iodine produced in the primary C–I bond fi...

Vibrational predissociation spectra and dynamics of small molecular clusters of H2O and HF

Experimental results are presented for the vibrational predissociation spectra in the frequency range 3000–4000 cm–1 for the species (HF)n and (H2O)n, n= 2–6, using molecular-beam techniques and a tunable infrared laser. The observed spectra show a dramatic change between the dimer and larger clusters which is thought to be a result of the cyclic structure of the trimer and larger clusters. The spectra are compared with calculated harmonic force constants of available intermolecular potentials to understand how these small, gas-phase clusters relate to the liquid and solid phases of HF and H2O. Additionally, the angular distributions of the predissociation products show that little energy appears as translational motion of the fragment molecules. This conclusion is consistent with recent theoretical models of the predissociation process. An upper limit of ca. 2 µs is observed for the lifetime of the vibrationally excited clusters.

Crossed Laser and Molecular Beam Study of Multiphoton Dissociation of C2F5Cl

Recent molecular beam experiments [1] have shown that infrared multiphoton dissociation of molecules mainly proceeds through the lowest available dissociation channel. Larger molecules may have the two (or more) lowest dissociation channels close in energy. When the laser energy fluence is adequate, the molecules can be excited above the dissociation energies of all these channels, with the branching rate into each available dissociation channel determined by statistical considerations since the excitation energy is expected to be randomized rapidly. In the case of C2F5Cl two competing dissociation channels exist: Cl atom elimination at 83 kcal/mole and C-C bond rupture at 97 kcal/mole. Although C-C bond rupture has appreciably higher dissociation energy, with three more rotational degrees of freedom in the fragments, the rate constant is expected to increase faster than Cl elimination when the excitation energy becomes higher.Consequently, the ratio of Cl elimination to C-C rupture will be a good indicator of the level of excitation. RRKM rate constants for dissociation into these channels are shown in Fig. 1.

Infrared Vibrational Predissociation Spectroscopy of Small Molecular Clusters

The structure and interaction of small molecular clusters have been the subject of extensive studies. Molecular beam electric resonance spectroscopy, Fourier-transform microwave spectroscopy and pressure induced infrared absorption spectra have provided information concerning the ground state structure and the intermolecular potential energy surface for a large number of binary systems. Recently, vibrational predissociation experiments using molecular beam techniques and tunable infrared lasers have measured the infrared absorption spectra and dynamical properties of many van der Waals and hydrogen bonded clusters.

Crossed Laser and Molecular Beams Studies of Multiphoton Dissociation

In 1973, Canadian [1] and Russian [2] groups reported the observation of visible luminescence from C02 laser-irradiated molecular gases (SiF4, NH3, C2F3Cl) at powers well below the breakdown threshold. The extremely fast rise-time and the pressure-dependence of one part of the luminescence suggested that the molecules might be absorbing tens of photons required for dissociation under collisionless conditions. The demonstration of isotopic selectivity in the multiphoton dissociation (MPD) of a variety of molecules [3] supports this conclusion. The alluring possibility that MPD might lead to mode-selective bond breaking in molecules has also been suggested [4].