Excited HBr Research Articles

Long time scale dynamics of vibrationally excited (HBr)n clusters.

We investigated the photodissociation dynamics of vibrationally excited HBr molecules and clusters. The species were generated in a molecular beam and excited with an IR laser to a = 1 vibrational state. A subsequent ultraviolet (UV)-pulse with 243 nm radiation photolysed the molecules to yield H-fragments, which were resonantly ionized by the same UV-pulse (2 + 1 REMPI) and detected in a velocity map imaging (VMI) experiment. We performed action spectroscopy to distinguish between two expansion regimes: (i) expansion leading to isolated HBr molecules and (ii) generation of large (HBr)n clusters. Photodissociation of isolated HBr ( = 1) molecules in particular J ro-vibrational states yielded faster H-fragments (by approximately 0.3 eV) with respect to the photodissociation of the ground state HBr ( = 0). On the contrary, the IR excitation of molecules in (HBr) n clusters enhanced the yield of the H-fragments UV-photodissociated from the ground-state HBr ( = 0) molecules. Our findings show that these molecules are photodissociated within clusters, and they are not free molecules evaporated from clusters after the IR excitation. Nanosecond IR-UV pump-probe experiments show that the IR-excitation enhances the H-fragment UV-photodissociation yield up to ∼100 ns after the IR excitation. After these long IR-UV delays, excitation of HBr molecules in clusters does not originate from the IR-excitation but from the UV-photodissociation and subsequent caging of HBr molecules in > 0 states. We show that even after ∼100 ns the IR-excited larger (HBr) n clusters do not decay to individual molecules, and the excitation is still present in some form within these clusters enhancing their UV-photodissociation.

Role of vibrationally excited HBr in a HBr/He inductively coupled plasma used for etching of silicon

In this work, the role of vibrationally excited HBr (HBr(vib)) is computationally investigated for a HBr/He inductively coupled plasma applied for Si etching. It is found that at least 50% of all dissociations of HBr occur through HBr(vib). This additional dissociation pathway through HBr(vib) makes the plasma significantly more atomic. It also results in a slightly higher electron temperature (i.e. about 0.2 eV higher compared to simulation results where HBr(vib) is not included), as well as a higher gas temperature (i.e. about 50 K higher than without including HBr(vib)), due to the enhanced Franck–Condon heating through HBr(vib) dissociation, at the conditions investigated. Most importantly, the calculated etch rate with HBr(vib) included in the model is a factor 3 higher than in the case without HBr(vib), due to the higher fluxes of etching species (i.e. H and Br), while the chemical composition of the wafer surface shows no significant difference. Our calculations clearly show the importance of including HBr(vib) for accurate modeling of HBr-containing plasmas.

Influence of collision energy and vibrational excitation on the dynamics for the H+HBr→H2+Br reaction

Quasi-classical trajectory (QCT) calculations of H + HBr → H2+Br reaction have been performed on a recently proposed ab initio potential energy surface. The reaction probability and integral cross section are found to be in fairly good agreement with the available quantum mechanical (QM) results on this surface. The behavior of reactivity is well consistent with properties of exothermic reaction. Once the energy of vibrational excited HBr is larger than the barrier height, the integral cross sections for the reaction diverge at very low collision energies close to the threshold, similarly to capture reaction. In addition, differential cross sections show that scattering of the product H2 shift from backward to forward directions as the collision energy and vibrational quantum number increase. All the theoretical findings are reasonably explained by the properties of the surface, as well as reactive mechanisms. A detailed dynamical study of reaction, H+HBr (v=0-3,j=0) → H2+Br, with QCT method has been performed. Both present QCT and available QM results for the HBr (v=0, j=0) have been found to be fairly consistent with each other. The reaction switches from activated to capture-type with increase of the quantum number v.

Infrared fluorescence from multiphoton dissociation of vinyl bromide: Emission from the products and the parent molecule

Abstract A time-resolved study of the infrared fluorescence emission produced in the infrared multiphoton dissociation of vinyl bromide at different experimental conditions has been carried out. Two different regimes, namely unimolecular and collisional, have been identified and characterized through the parent vinyl bromide and the HBr and C 2 H 2 photoproducts emissions. In the pressure interval studied, the formation of HBr has been determined to be mainly unimolecular, while the C 2 H 2 is produced through a collisionally assisted photodecomposition process. The collisional regime is explained in terms of the 3-centre elimination channel while the unimolecular regime is ascribed to the 4-centre mechanism. Relaxation rate constants for both the vibrationally excited HBr by vinyl bromide and by Ar and for vibrational excited VBr by unexcited VBr, are reported.

Velocity map imaging of HBr photodissociation in large rare gas clusters

We have implemented the velocity map imaging technique to study clustering in the pulsed supersonic expansions of hydrogen bromide in helium, argon, and xenon. The expansions are characterized by direct imaging of the beam velocity distributions. We have investigated the cluster generation by means of UV photodissociation and photoionization of HBr molecules. Two distinct features appear in the hydrogen atom photofragment images in the clustering regime: (i) photofragments with near zero kinetic energies and (ii) "hot" photofragments originating from vibrationally excited HBr molecules. The origin of both features is attributed to the fragment caging by the cluster. We discuss the nature of the formed clusters based on the change of the photofragment images with the expansion parameters and on the photoionization mass spectra and conclude that single HBr molecule encompassed with rare gas "snowball" is consistent with the experimental observations.

5422578 Magnetic resonance probehead

A time-resolved study of the infrared fluorescence emission produced in the infrared multiphoton dissociation of vinyl bromide at different experimental conditions has been carried out. Two different regimes, namely unimolecular and collisional, have been identified and characterized through the parent vinyl bromide and the HBr and C2H2 photoproducts emissions. In the pressure interval studied, the formation of HBr has been determined to be mainly unimolecular, while the C2H2 is produced through a collisionally assisted photodecomposition process. The collisional regime is explained in terms of the 3-centre elimination channel while the unimolecular regime is ascribed to the 4-centre mechanism. Relaxation rate constants for both the vibrationally excited HBr by vinyl bromide and by Ar and for vibrational excited VBr by unexcited VBr, are reported.

Quantal sudden calculations on the exchange reactions of vibrationally excited HBr ( v = 1) with H and D atoms in three dimensions

Three-dimensional calculations on the vibrationally excited exchange reactions D + BrH( v = 1) → DBr( v′) − H and H − BrH( v = 1) → HBr( v′) + H are reported. Two quantal sudden techniques are used to calculate vibration-rotation, integral and differential cross sections, and rate coefficients. In the first (ESA CSA) method, the energy and centrifugal sudden approximations are applied to the entrance and exit channel hamiltonians respectively. The second, less accurate, technique is the infinite-order-sudden approximation (IOSA). A diatomics-in-molecules potential energy surface is used which has been optimised to reproduce the experimental rate coefficient for the D + BrH( v = 0) − DBr + H reaction. The IOSA and ESA CSA results are in good agreement. It is found that the v = 1 → v′ = 0 reactions have quite large rate coefficients. New calculations are also reported for the D(H) + BrH( v = 0) exchange reactions. For the v = 0 reaction, it is found that the H-atom rate coefficients are larger than those for the D-atom reaction due to tunnelling, while the reverse is true for the BrH( v = 1) reactions, where tunnelling is less important. The average energy for product translation, and differential cross section, for the D + BrH( v = 0) reaction, obtained in a previous molecular-beam experiment, are in good agreement with our ESA CSA results, computed for similar collisional conditions.

Chemical production of vibrationally excited HBr for a continuous wave laser

Bromine has been added to the hydrogen cavity fuel of a cw supersonic mixing HF chemical laser. The hydrogen atoms produced in the F+H2→HF(v⩽3)+H reaction are used to continuously produce vibrationally excited HBr by the reaction H+Br2→HBr(v⩽6)+Br. Infrared chemiluminescence measurements indicate unsaturated gain coefficients of about (2±1) ×10−3 cm−1 for J=4 through J=8 for v=3, 2, and 1 levels of HBr. The current 3-cm gain length is not sufficient for lasing without the use of high-reflectivity mirrors, but a scaling to 18-cm gain length would be appropriate for a cw HBr chemical laser operating in the 4.0- to 4.5-μm region.

A purely chemical HBr laser

Purely chemical laser action in HBr has been achieved. Chlorine atoms which had been produced by the reaction of NO with ClO2 were reacted with molecular bromine to form bromine atoms which, in turn, reacted with HI to form vibrationally excited HBr. Multiline HBr output power of 0.58 W was obtained with lasing occurring on three lines from the v=1→0 vibrational band, P1−0(4), P1−0(5), and P1−0(6).

Reaction of vibrationally excited HBr and I

The rate of the reaction HBr(ν) + I → HI + Br increases by at least a factor of 10 9 when HBr(ν=0) is excited to HBr(ν ⩾ 2). The increase is observed when the reacting level is directly excited by sequential laser excitation or when it is populated by collisional excitation. Bromine isotopic enrichment in BrI, the product of a subsequent reaction, was observed after isotopically selective excitation.

Vibrational Relaxation of HBr(ν″ = 1) by HCl at 300 K

RECENT interest in chemical lasers has been strongly directed towards systems which give rise to vibrationally excited hydrogen halide molecules1–7. The efficiency of such systems will clearly depend on the rate at which vibrational energy is removed from the products of the chemical reaction, and considerable effort has been made to obtain the relevant rate data. Airey8 has described a pulsed laser system based on the reaction9,10 and has discussed the various energy transfer processes which may be involved. Moore et al. and Airey (see ref. 11) have further used reaction (1) to obtain rate data for the process. We have independently undertaken an extensive study of energy transfer processes involving the hydrogen bromide molecule using rather different techniques to monitor the vibrationally excited species and wish to report here rate data for the vibrational relaxation of HBr(ν″ = 1) by HCl. In our system, vibrationally excited HBr is produced by the fast reaction9,10 following the flash photolysis of HBr (pHBr = 33.3 Nm−2) in the presence of Br2 ( = 4.1 Nm−2), and excess Ar (pAr = 13.3 k Nm−2) to prevent a significant rise above room temperature (T = 300 K). Near resonant exchange processes rapidly reduced the initial vibrational distribution in HBr to a condition where a non-Boltzmann distribution in HBr(ν″ = 1) is observed. Relaxation from this level to ν″ = 0 can then be conveniently monitored, using kinetic absorption spectroscopy in the vacuum ultraviolet12, to observe the (0,1) transition of the strongly predissociated system connecting the ground state with “state [6]” (ν0 = 70,526 cm−1) (refs. 13, 14).