Electronic transition chemical lasers

An extremely metastable excited state is not itself a suitable laser candidate because the optical gain is directly proportional to the radiative rate. One is faced with a dilemma If the only states diat can be efficiently produced are extremely long lived, how can one hope to build a laser A possible solution to this problem is found in the only electronic transition chemical laser yet demonstrated, the chemical oxygen iodine laser (COIL). In the COIL chemically produced, highly metastable 02( A) resonantly transfers energy to atomic iodine, and an inversion is produced between the Pi/2 and P3/2 iodine levels and atomic lasing occurs at 1.315 im. This laser was predicted by Derwent and Thrush [11] in 1972 and was demonstrated by McDermott et al in 1977 [12]. This device has been described in numerous papers [12-16], and we do not discuss it further. It clearly demonstrates the concept of a transfer-laser and may serve as a model for future visible lasers using this two step approach. 

The important features of a successful cw electronic transition chemical laser capable of operation at visible wavelengths may be summarized ... 

Pulsed electronic transition chemical laser operation appears to be possible in a much wider class of reaction systems than those defined by the foregoing considerations. Reaction initiation in premixed reagents by pulsed techniques can provide population inversions on time scales short compared with fluid mixing times and relaxation times for lower laser states. Such techniques include photolysis, laser-induced dissociation, electrical discharge production of atoms or metastables, and shock-induced dissociation and pyrolysis. Such lasers are subject to the pumping rate condition of Figure 3.21 which is much less restrictive than the condition for diffusive mixing... 

As the considerations of Section 3.4.1 demonstrate, an important requirement for either pulsed or cw electronic transition chemical laser operation is the achievement of large metal atom or free-radical concentrations. Moreover, these reagent concentrations must be made available in a manner that ensures that the chemical pumping rate of the upper laser level is large enough compared with the rates of the competing processes of collisional quenching and radiative, decay that an upper level population is created sufficient to exceed laser threshold requirements. 

A more extensive consideration of the prospects for electronic transition chemical lasers has been presented see L. E. Wilson et al. New Gas Lasers Committee Report on Electronic Transition Chemically and Electronically Excited Lasers, Technical Report AFWL-TR-73-60 (U.S. Air Force Weapons Laboratory, Kirtland AFB, New Mexico, May 1973). 

In the course of studies toward the development of an electronic transition chemical NF laser, that is, production of high densities of electronically excited NF radicals, scale-up studies of NF(a) in a subsonic laser device [20, 27] and in a combustor-driven flow facility [2, 3] succeeded in steady-state densities of about 10 " molecules/cm and thus in an increase by a factor of 10" over earlier flow-tube studies [28]. Scaling to even larger concentrations of about 6x10 molecules/cm was demonstrated in a supersonic flow [29]. For a reasonable laser gain, however, densities on the order of 10 molecules/cm would be required because of the long radiative lifetime of NF(a) [2, p. 109]. This estimate, however, was based on t(a A) 0.7 s which is smaller than the more recent value by a factor of 8 (cf. p. 287). 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

From the point of view of associative desorption, this reaction is an early barrier reaction. That is, the transition state resembles the reactants.46 Early barrier reactions are well known to channel large amounts of the reaction exoergicity into product vibration. For example, the famous chemical-laser reaction, F + H2 — HF(u) + H, is such a reaction producing a highly inverted HF vibrational distribution.47-50 Luntz and co-workers carried out classical trajectory calculation on the Born-Oppenheimer potential energy surface of Fig. 3(c) and found indeed that the properties of this early barrier reaction do include an inverted N2 vibrational distribution that peaks near v = 6 and extends to v = 11 (see Fig. 3(a)). In marked contrast to these theoretical predictions, the experimentally observed N2 vibrational distribution shown in Fig. 3(d) is skewed towards low values of v. The authors of Ref. 44 also employed the electronic friction theory of Tully and Head-Gordon35 in an attempt to model electronically nonadiabatic influences to the reaction. The results of these calculations are shown in... 

Raman spectroscopy is primarily useful as a diagnostic, inasmuch as the vibrational Raman spectrum is directly related to molecular structure and bonding. The major development since 1965 in spontaneous, c.w. Raman spectroscopy has been the observation and exploitation by chemists of the resonance Raman effect. This advance, pioneered in chemical applications by Long and Loehr (15a) and by Spiro and Strekas (15b), overcomes the inherently feeble nature of normal (nonresonant) Raman scattering and allows observation of Raman spectra of dilute chemical systems. Because the observation of the resonance effect requires selection of a laser wavelength at or near an electronic transition of the sample, developments in resonance Raman spectroscopy have closely paralleled the increasing availability of widely tunable and line-selectable lasers. 

The complex quantity, y6br = e (y(3)r) + i Im (x r), represents the nuclear response of the molecules. The induced polarization is resonantly enhanced when the Raman shift wp — ws matches the frequency Qr of a Raman-active molecular vibration (Fig. 6.1A). Therefore, y(3)r provides the intrinsic vibrational contrast mechanism in CRS-based microscopies. The nonresonant term y6bnr represents the electronic response of both the one-photon and the two-photon electronic transitions [30]. Typically, near-infrared laser pulses are used to prevent the effect of two-photon electronic resonances. With input laser pulse frequencies away from electronic resonances, y(3)nr is independent of frequency and is a real quantity. It is important to realize that the nonresonant contribution to the total nonlinear polarization is simply a source for an unspecific background signal, which provides no chemical contrast in some of the CRS microscopies. While CARS detection can be significantly effected by the nonresonant contribution y6bnr [30], SRS detection is inherently insensitive to it [27, 29]. As will be discussed in detail in Sects. 6.3 and 6.4, this has major consequences for the image contrast mechanism of CARS and SRS microscopy, respectively. 

Laser Control of Chemical Dynamics. I. Control of Electronic Transitions by Quadratic Chirping... 

In this sense, the control of electronic transitions of wavepackets using short quadratically chirped laser pulses of moderate intensity is a very promising method, for two reasons. First, only information about the local properties of the potential energy surface and the dipole moment is required to calculate the laser pulse parameters. Second, this method has been demonstrated to be quite stable against variations in pulse parameters and wavepacket broadening. However, controlling of some types of excitation processes, such as bond-selective photodissociation and chemical reaction, requires the control of wavepacket motion on adiabatic potential surfaces before and/or after the localized wavepacket is made to jump between the two adiabatic potential energy surfaces. 

By combining the control of electronic transitions of wavepackets using quadratically chirped laser pulses with semiclassical optimal control [34,35] on a single adiabatic surface, we should be able to establish an effective methodology for controlling the dynamics of large-dimensional chemical and biological systems. 

S. Zou, A. Kondorskiy, G. Mil nikov and H. Nakamura, Laser control of chemical dynamics. L. Control of electronic transitions by quadratic chirping. In Progress in Ultrafast intense Laser Science (Springer, Berlin Heidelberg New York, 2005)... 

The best prospect for creating a chemically driven electronic transition laser is to use a transition where the upper laser level is metastable. Chemical methods that generate high yields of certain metastables have been developed, but so far it has not been possible to achieve lasing on the primary products. For example, inverted populations of the metastables NF(a A) and NCl(a A) can be generated by chemical means, but the a A —... 

In this chapter, I will consider chemically pumped electronic transition lasers that are based on energy transfer from 02(a) or NX(a) metastables, focusing on the energy transfer and reaction kinetics of these devices. As the... 

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