Collisional population transfer

The states arising from the. .. Ip Ss configuration of Ne have very nearly the same energy as that of the 2 Sq state of He so that collisional energy transfer results in efficient population of these Ne states. Similarly, the states arising from the. .. configuration... 

Some of our recent studies of LIF on OH in flames demonstrate the close connection between current work in other areas of physical chemistry—in this case, state-to-state collisional energy transfer—and the development of diagnostic tools for combustion. In these experiments, measurements are made of the collisional redistribution of excited state population following laser excitation of OH to individual levels, in an atmospheric pressure flame. 

A[N (e) + (e)], the net population transfer into or out of levels le and 2e during the laser pulse, will be nearly zero if the laser pulse length tl is less than or comparable to the characteristic collisional transfer time tc - (Q23+T24)- Ql4 > simply because few collisions will occur during the laser pulse. 

Fig. 4. Spectra showing rovibrational level state preparation and collisional energy transfer. The upper trace demonstrates clean preparation of the l2(.X ), v = 23, J = 57 level under collision-free conditions. The middle and lower traces show the effects of rovibrational energy transfer induced by collisions with H2O (middle trace) and Ar (lower trace). The peaks marked with asterisks originate from levels populated by vibrational energy transfer.

Although flames are convenient sources of MOH molecules, they suffer from serious drawbacks for spectroscopic and dynamical studies. The high temperature ( 2000 K) of flames causes numerous vibrational and rotational levels to be populated resulting in very dense spectra. The high pressure (1 atm) broadens the rotational lines (>0.1 cm ) and increases the overlap of the lines. In addition, resonant laser-induced fluorescence is difficult to detect because of quenching and the overwhelming presence of nonresonant fluorescence caused by rapid collisional energy transfer. The luminescence of the flame itself also interferes with measurements. 

B. Glushko, B. Kryzhanovsky Radiative and collisional damping effects on efficient population transfer in a three-level system driven by two delayed laser pulses, Phys. Rev. A 46 (1992) 2823. 

A third aspect of LIF is the spectroscopic study of collision processes. If the excited molecule is transferred by inelastic collisions from the primarily excited level (n[, Jj ) into other rovibronic levels, the fluorescence spectrum shows new lines emitted from these collisionally populated levels which give quantitative information on the collision cross sections (Sect. 8.4). 

In addition to the non-radiative quenching mentioned further above, an addition collisional energy transfer process can be observed, namely the transfer from the laser-excited level to neighbouring quanmm levels within the excited-state manifold. Hence, under the right conditions, one observes lines from levels that were not directly populated by the laser excitation. 

Both linear and nonlinear Raman spectroscopy can be combined with time-resolved detection techniques when pumping with short laser pulses [8.781. Since Raman spectroscopy allows the determination of molecular parameters from measurements of frequencies and populations of vibrational and rotational energy levels, time-resolved techniques give information on energy transfer between vibrational levels or on structural changes of short-lived intermediate species in chemical reactions. One example is the vibrational excitation of molecules in liquids and the collisional energy transfer from the excited vibrational modes into other levels or into translational energy of the collision partners. These processes proceed on picosecond to femtosecond time scales [8.77,8.79]. 

In 1934, N. Semenov, in his book on chain reactions [2] strongly emphasized the role of the collisional energy transfer in gas-phase chemical kinetics, particularly paying attention to different kind of molecular energy, electronic, vibrational, rotational and translational. However, it was not until the work by Landau and Teller in 1936 [3] when it was realized that the collisional energy transfer should be described in terms of kinetics of populations of individual energy levels. Later on, the discussion of the energy transfer become indispensable sections of comprehensive texts on chemical kinetics as exemplified by the Kondratiev book [4]. 

A CO2 laser operates on the emission bands between vibrational combination states generating emission on discrete rovibrational transitions in the i>i and 2 2 3 bands, centred around 10.6 and 9.6 pm, respectively. Population inversion is achieved by collisional energy transfer from plasma-excited N2 to CO2, usually in a mixture with He. A particular rovibrational emission line can be selected using a rotatable diffraction grating incorporated in the laser cavity. CO2 lasers can achieve very high continuous-wave (cw) power levels of up to 100 W from commercially available systems. In addition, CO2 lasers are robust, narrow-bandwidth and low-cost systems well able to induce IRMPD, but a disadvantage is clearly its limited tunability. It should be noted that fixed-frequency CO2 lasers are used routinely in commercial MS platforms to induce dissociation as an alternative to CID. 

The difference between the two spectroscopic approaches stems from the means by which the excited state is populated. In fluorescence, absorption of radiation at the proper frequency is used for excitation in emission spectroscopy, collisional energy transfer with other atoms, molecules, ions or electrons produces the excited-state population. 

Before proceeding, an important note must be made. In literature, two different but fully equivalent approaches have been taken in s.e. The first approach considers a cell that contains (unknown) numbers of donors and acceptors No and NA. When energy transfer takes place (be it from collisional encounters or because a stable population of FRET pairs exist with FRET efficiency E) this diminishes the effective number of emitting donors with Ns [3] that is, the FRET efficiency for this population is unity. Thus, the residual donor emission results from (No — Ns) unquenched donor molecules, and the Ns population emits only sensitized emission. This approach is intuitive in case no assumptions are being made on the presence of a stable population of FRET pairs or on the magnitude of E in a donor-acceptor complex. 

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... 

Of a special astronomical interest is the absorption due to pairs of H2 molecules which is an important opacity source in the atmospheres of various types of cool stars, such as late stars, low-mass stars, brown dwarfs, certain white dwarfs, population III stars, etc., and in the atmospheres of the outer planets. In short absorption of infrared or visible radiation by molecular complexes is important in dense, essentially neutral atmospheres composed of non-polar gases such as hydrogen. For a treatment of such atmospheres, the absorption of pairs like H-He, H2-He, H2-H2, etc., must be known. Furthermore, it has been pointed out that for technical applications, for example in gas-core nuclear rockets, a knowledge of induced spectra is required for estimates of heat transfer [307, 308]. The transport properties of gases at high temperatures depend on collisional induction. Collision-induced absorption may be an important loss mechanism in gas lasers. Non-linear interactions of a supermolecular nature become important at high laser powers, especially at high gas densities. 

One of the more well studied collision processes involving Rydberg atoms is collisional angular momentum mixing, or i mixing, the collisional transfer of population among the nearly degenerate states of the same n.28 The process has... 

The depopulation cross sections of the Rb nd states of 25 < n < 40 are 1000 A2, which is the same as the cross section of the Rb ns state if the ns —> (n - 3)1,1 > 3 contribution is subtracted. For the Rb nd states the calculated contribution of the scattering of the nd state to nl S 3 and (n—1)1 s 3 states with no change in the rotational state of the CO is <100 A2, so 90% of the cross section is due to the inelastic transitions leading to rotational excitation. Presumably it is because the resonant transfer accounts for 90% of the observed cross section that the structure in the cross section is more visible in the nd cross sections than in the ns cross sections. For both the ns and nd states minimal collisional ionization is observed and calculated in this n range, principally because there are too few CO molecules with energetic enough A/ = -1 rotational transitions. For example, only CO 7 > 18 states can ionize an n = 42 Rydberg state by a A7 = -1 transition, and only 3% of the rotational population distribution is composed of 7 > 18 states. 

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