Collisional effects

Crosley D R 1981 Collisional effects on laser-induced fluorescence Opt. Eng. 20 511-21... 

When collisional effects are negligible the second component in (3.8) vanishes, and J becomes a constant. The solution obtained in this case is used in Eq. (3.4). If collisions occur they change J and frustrate the vibrational phase simultaneously. Nevertheless, the processes are usually considered to be statistically independent ... 

St. Pierre A. G., Steele W. A. Collisional effects upon rotational correlations of symmetrical top molecules. J. Chem. Phys. 57, 4638-48 (1972). 

Fluorescence and collisional excitation, arising primarily from the metastability of the 23S level (see Fig. 4.9), in which consequently a high population accumulates which can cause additional emission from lines such as X 4471, X 5876 by either collisional excitation or radiative transfer effects following absorption of higher lines in the 23S — n3P series. The singlet line X 6678 can also be enhanced by collisional excitation from 23S. The collisional effects can be calculated from the known electron temperature and density, and are quite small at... 

In Eq. (10), E nt s(u) and Es(in) are the s=x,y,z components of the internal electric field and the field in the dielectric, respectively, and p u is the Boltzmann density matrix for the set of initial states m. The parameter tmn is a measure of the line-width. While small molecules, N<pure solid show well-defined lattice-vibrational spectra, arising from intermolecular vibrations in the crystal, overlap among the vastly larger number of normal modes for large, polymeric systems, produces broad bands, even in the crystalline state. When the polymeric molecule experiences the molecular interactions operative in aqueous solution, a second feature further broadens the vibrational bands, since the line-width parameters, xmn, Eq. (10), reflect the increased molecular collisional effects in solution, as compared to those in the solid. These general considerations are borne out by experiment. The low-frequency Raman spectrum of the amino acid cystine (94) shows a line at 8.7 cm- -, in the crystalline solid, with a half-width of several cm-- -. In contrast, a careful study of the low frequency Raman spectra of lysozyme (92) shows a broad band (half-width 10 cm- -) at 25 cm- -,... 

Given the significance of collisional effects in solution, we introduce the simplest of models for relaxation and concomitant decoherence. That is, the equation motion of px t) in the energy representation is given the form i... 

Thus we see that, although collisional effects do reduce the degree of controlji relative to the collision-free case, saturation pumping of superposition in the bichro- ) matic control scenario can be used to overcome collisional effects up to somejli reasonable temperature. 

Finally we note that studies of control in solution [186, 187] indicate that control in the presence of collisional effects is indeed possible. For example, coherent control of the dynamics of I3 in ethanol and acetonitrile has been demonstrated. Specifically, I3 was excited with a 30-fs ultraviolet (UV) laser pulse to the first excited state, The resultant wave function was comprised of a localized wave function on the ground electronic state and a corresponding depletion of wave function density, that is, a hole, on the ground electronic state. In this instance the target of the control was the nature of the spectrum associated with the coherences associated with the symmetric stretch. By manipulating various attributes of the exciting pulse (intensity, frequency, and chirp of the excitation pulse), aspects of the spectrum were controlled, despite the decoherence associated with collision effects. 

Here we show that an application of bichromatic control (Section 3.1.1) allows us to control both the real and imaginary parts of the refractive index. In doing so we consider isolated molecules [213, 214], or molecules in a very dilute gas, where collisional effects can be ignored and time scales over which radiative decay occurs can be ignored. 

A typical problem of interest at Los Alamos is the solution of the infrared multiple photon excitation dynamics of sulfur hexafluoride. This very problem has been quite popular in the literature in the past few years. (7) The solution of this problem is modeled by a molecular Hamiltonian which explicitly treats the asymmetric stretch ladder of the molecule coupled implicitly to the other molecular degrees of freedom. (See Fig. 12.) We consider the the first seven vibrational states of the mode of SF (6v ) the octahedral symmetry of the SF molecule makes these vibrational levels degenerate, and coupling between vibrational and rotational motion splits these degeneracies slightly. Furthermore, there is a rotational manifold of states associated with each vibrational level. Even to describe the zeroth-order level states of this molecule is itself a fairly complicated problem. Now if we were to include collisions in our model of multiple photon excitation of SF, e wou d have to solve a matrix Bloch equation with a minimum of 84 x 84 elements. Clearly such a problem is beyond our current abilities, so in fact we neglect collisional effects in order to stay with a Schrodinger picture of the excitation dynamics. 

Besides the initial object of CARS as a tool for the determination of molecular energy levels and its application to combustion studies and related areas, there are several other scientific applications of academic interests. These include spectral lineshape studies, collisional effects, Dicke narrowing, nonadditivity effects, and motional narrowing. CARS turned out to be an excellent method for these studies (Berger et al., 1992 and references therein). 

J. Berrue, A. Chave, B. Dumon, and M. Thibeau. Density dependence of light scattering from nitrogen Permanent and collisional effects and their interferences. Canad. J. Phys., 59 1510-1513(1981). 

Pressure studies indicated a second power dependence of the Ha ion current on the pressure in the einzel lens chamber. Since similar effects were observed with Ar, Ne, H2, Kr and Xe, Schnitzer and Anbar conclude that the effect is not one solely involving H2. They concluded from these studies that the collisional effect is relatively insignificant and that the primary effect of the pressure is to enhance the initial production of ions and/or their extraction from the source. 

J.A. Loo, H. R. Udseth, R.D. Smith, Collisional effects on the charge distribution of ions from large molecules, formed by ESI-MS, Rapid Commun. Mass Spectrom., 2 (1988)207. 

Consider first the force power spectrum. At modest bath densities this is to be derived for particle pairs initially at R moving inward. The strong repulsive potential for R < Rp leads to a power sjjectrum 8/iBMk7BBM(o>) of a form discussed in Section III C (with di = 0 because the trajectories begin at Rp, not Rj, and do not exjjerience the acceleration effect). The delay in time between initiation at Rp and recoil is so small that collisional effects are... 

In a dense medium, collisional effects as well as time and space fluctuations of multipolar molecular fields will in general lead to changes in the molecular polarizability tensor. Then, according to Eq. (1), the pair polarizability tensor of two molecules A and B, Aap, acquires an incremental collision-induced... 

The photoionization threshold values listed in column 4 are all lower by some 0.002 to 0.005 eV (15-30 cm l). Similar differences were found in uranium and neptunium and remain unexplained. Electric fields from the ion optics (field ionization) and collisional effects are possible explanations. In all cases, the Rydberg convergence limits are the most accurate and they are the preferred values. 

Comprehensive studies have been carried out on the MPD of hexafluoroacetone as a function of laser fluence, frequency and substrate pressure, and the influence of collisional effects on the formation of CF2 in the MPD of CF2CFCI and CFjHCl has been experimentally measured and theoretically modelled. Fluence dependences of the HF laser-induced decomposition of 2,2,2-trifluoroethanol, and the 9.4 pm CO2 laser MPD of C2F3C1 have been reported, and triethylphosphite joins the increasing list of molecules dissociated by CO2 laser radiation. ... 

Considerable experimental effort has been aimed at elucidating the collision-free unimolecular dynamics of excited molecules. Processes of interest include the dynamics of highly excited vibrational states, which have been reached by multiphoton absorption, and the various electronic relaxation processes that can occur in electronically excited states of moderate to large molecules, etc. The idealized collision-free limit is approached either by extrapolating data to the limit of zero pressure or by performing experiments in molecular beams. Alternatively, estimates of expected collisional effects are made by using collision cross-sections that are computed from hard-sphere collision rates. These estimates are then utilized to determine whether the experiments are performed in the collision-free domain. 

A previous review provides a description of the theory of electronic relaxation in polyatomic molecules with particular emphasis on the vibronic state dependence of radiationless transition rates. A sequal review considers the general question of collisional effects on electronic relaxation, while the present one covers only the special phenomenon of collision-induced intersystem crossing. It departs from the other collisional effects review in presenting only a qualitative description of the theory the full theoretical details can be obtained from the previous review and the original papers.As a review of the basic concepts of radiationless transitions theory is necessary as a prelude to a discussion of collision-induced intersystem crossing, considerable overlap exists between this section and Section II of the previous collision effects review. However, since many concepts from radiationless transition theory, such as the nature and criteria for irreversible decay, the role of the preparation of the initial state, the occurrence of intramolecular vibrational relaxation, etc. pervade the other papers on laser chemistry in these volumes, it is useful to recall the primary results of the theory of electronic relaxation in isolated molecules and its relevance to the material in the present volume as well as to this review. 

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