Vibration-rotation excitation

Figure 3 demonstrates the simplifications in the spectrum of an optimized laser pulse that can be achieved through the application of the sifting technique [see Fq. (7)]. The excitation efficiency of the pulse is only minimally reduced due to the additional restrictions imposed in the sifting procedure. The example used in this case is for a vibrational-rotational excitation process, H2(v = 0,7 = 0) H2(v =1,/ = 2). 

A number of inelastic scattering processes have been carried out which may be cited as examples of vibration-rotation excitation in which ions other than H+ or H2 were the projectiles. 

In each case, the product molecule HCl is vibrationally excited. The fraction of the energy liberated in these reactions which appears as vibration-rotational excitation of HCl is greater for attack by Cl than for attack by the much lighter H atom. At least part of the decreased vibration in HCl from H -F CI2 is explicable by the fact that the light attacking atom approaches the centre of mass of the system too rapidly to allow energy release by repulsion between the two Cl atoms whilst H and Cl are still at an extended separation. ... 

It has to be kept in mind that the translational and internal energy distributions of the desorbing species are of great importance. Therefore, typical detection tools are TOFMS to derive velocity and angular distributions, and either LIF or REMPI are used to determine internal energy distributions. Both LIF and REMPI provide information on the vibration-rotation excitation of photodesorbing species or fragments. 

The first experiments were carried out in 1983 [13.99,13.100]. The H atoms were produced by photodissociation of HJ molecules in an effusive beam using the fourth harmonics of Nd YAG lasers. Since the dissociated iodine atom is found in the two fine-structure levels /(P1/2) and /(P3/2), two groups of H atoms with translational energies kin = 0.55 eV or 1.3 eV in the center-of-mass system H-f-D2 are produced. If the slower H atoms collide with D2 they can reach vibrational-rotational excitation energies in the product molecule up to (u = 1, / = 3), while the faster group of H atoms can populate levels of HD up to (v = 3, J = 8). The internal-state distribution of the HD molecules can be monitored either by CARS (Sect. 8.3) or by resonant multiphoton ionization [13.99]. Because of their fundamental importance, these measurements have been repeated by several groups with other spectroscopic techniques that have improved signal-to-noise ratios [13.101]. 

Absorptions that occur in the region from 1250-500 cm usually arise from a complex vibrational-rotational excitation of the entire molecule. This portion of the spectrum is typically unique for a particular compound and is aptly described as the fingerprint region. Although the IR spectra of similar molecules may be... 

When the experimental resolution profile was incorporated into the theoretical results, the measured and calculated intensities for Li -N, came into remarkably good agreement, as is demonstrated by fig. 10 for E = 8.4 eV. 6 - 37° and 73°. The semiclassical TCFs gave excellent results throughout the wide ranges E and 0 measured by experiments (4 eV s < 17 eV and 35° < 6f s 105°), and they explained the complicated structures of the measured cross sections [62], For instance, fig. 11 shows the distributions of transferred rotational energy corresponding to each vibrational transition n. One thus sees that the narrow peak measured near e = 0 arises from purely rotational excitation n = 0), while the broad peak centered near f = 1.2eV is due to combined vibrational-rotational excitations. 

Neglect of these particular operators removes the coupling in basis functions with different Q. quantum numbers and drastically reduces the size of die basis set[7,25]. This is the centrifugal sudden approximation (CSA) and is often known as the coupled-states approximation, although that is a term we prefer not to use as it is easily confused with "close-coupling". The basis set expansion for a CSA calculation of vibrational-rotational excitation cross sections would thus be... 

Unfortunately, most polyatomic molecules have rotor constants that are too small to enable converged CSA cross sections to be obtained for vibrational-rotational excitation as the required rotational basis sets will be large since so many rotational channels will be energetically open at typical collision energies. Thus further approximations are required. 

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