Photodissociation with lasers

The general theory for the absorption of light and its extension to photodissociation is outlined in Chapter 2. Chapters 3-5 summarize the basic theoretical tools, namely the time-independent and the time-dependent quantum mechanical theories as well as the classical trajectory picture of photodissociation. The two fundamental types of photofragmentation — direct and indirect photodissociation — will be elucidated in Chapters 6 and 7, and in Chapter 8 I will focus attention on some intermediate cases, which are neither truly direct nor indirect. Chapters 9-11 consider in detail the internal quantum state distributions of the fragment molecules which contain a wealth of information on the dissociation dynamics. Some related and more advanced topics such as the dissociation of van der Waals molecules, dissociation of vibrationally excited molecules, emission during dissociation, and nonadiabatic effects are discussed in Chapters 12-15. Finally, we consider briefly in Chapter 16 the most recent class of experiments, i.e., the photodissociation with laser pulses in the femtosecond range, which allows the study of the evolution of the molecular system in real time. 

Letokhov, V. S. (1969). On the possibility of isotope separation by resonant atomic photoionization and molecular photodissociation with laser radiation. Report, of Lebedev Physical Institute, Nov. 1969. (Pubhshed in preprint No. 1 (1979) of the Institute of Spectroscopy, USSR Academy of Sciences, pp. I 54). 

The existence of isotope shifts and of tunable lasers with narrow Hnewidth leads to the possibHity of separating isotopes with laser radiation (113,114). This can be of importance, because isotopicaHy selected materials are used for many purposes in research, medicine, and industry. In order to separate isotopes, one needs a molecule that contains the desired element and has an isotope shift in its absorption spectmm, plus a laser that can be tuned to the absorption of one of the isotopic constituents. Several means for separating isotopes are avaHable. The selected species may be ionized by absorption of several photons and removed by appHcation of an electric field, or photodissociated and removed by chemical means. 

In spite of the fact that in alkali vapors, which contain about 1 % diatomic alkali-molecules at a total vapor-pressure of 10 torr, the atoms cannot absorb laser lines (because there is no proper resonance transition), atomic fluorescence lines have been observed 04) upon irradiating the vapor cell with laser light. The atomic excited states can be produced either by collision-induced dissociation of excited molecules or by photodissociation from excited molecular states by a second photon. The latter process is not improbable, because of the large light intensities in the exciting laser beam. These questions will hopefully be solved by the investigations currently being performed in our laboratory. 

Escape probability of iodine atoms formed by photodissociation of iodine molecules with laser light pulses ( 530 nm wavelength, 30 ps duration) in various solvents. 

While one might expect that the techniques developed for photodissociation studies of closed-shell molecules would be readily adaptable to free radicals, this is not the case. A successful photodissociation experiment often requires a very clean source for the radical of interest in order to minimize background problems associated with photodissociating other species in the experiment. Thus, molecular beam photofragment translation spectroscopy, which has been applied to innumerable closed-shell species, has been used successfully on only a handful of free radicals. With this problem in mind, we have developed a conceptually different experiment [4] in which a fast beam of radicals is generated by laser photodetachment of mass-selected negative ions. The radicals are photodissociated with a second laser, and the fragments are detected in coinci-... 

D. M. Neumark We are currently carrying out somewhat different femtosecond experiments in which time-resolved photoelectron spectroscopy is used to probe the photodissociation dynamics of negative ions. In these experiments, an anion is photodissociated with a femtosecond laser pulse. After a time delay, the dissociating anion is pho-todetached with a second femtosecond pulse and the resulting photoelectron spectrum is measured. The photoelectron spectrum as a function of delay time provides a detailed probe of the anion photodissociation dynamics. First results have recently been obtained for the photodissociation of I2. 

There has been a flurry of recent activity in the study of the photodissociation dynamics of this molecule (186,187,188). Hermann and Leone used the infrared luminescence technique with a circular variable filter to determine the IR emission as a function of frequency when this molecule was photolyzed with lasers at 248 and 266 nm. From their results, they were able to show that the umbrella bending V2 mode of the CH3 radical was the only mode of CH3 that was excited in the photofragmentation of CH3I. This is in accord with the idea that the photodissociation of this molecule is unusually simple, and involves primarily the scission of the C-I bond with the simultaneous relaxation of the pyramidal structure of the CH3 part of the molecule into its final planar form. The data are used to obtain a vibrational distribution of the CH3 radical that peaks at v" = 2 and extends all the way out to the v" = 10 level. 

Output from both gated continuous wave and pulsed carbon dioxide lasers has been used to desorb ions from surfaces and then to photodissociate them in a Fourier transform ion cyclotron resonance mass spectrometer. Pulsed C02 laser irradiation was most successful in laser desorption experiments, while a gated continuous wave laser was used for a majority of the successful infrared multiphoton dissociation studies. Fragmentation of ions with m/z values in the range of 400-1500 daltons was induced by infrared multiphoton dissociation. Such photodissociation was successfully coupled with laser desorption for several different classes of compounds. Either two sequential pulses from a pulsed carbon dioxide laser (one for desorption and one for dissociation), or one desorption pulse followed by gated continuous wave irradiation to bring about dissociation was used. 

Fig. 19. Radial distribution functions f(r) and f(r) derived from the molecular scattering curves for gaseous CHJ2 at different delay times between the photodissociating fs laser pulse (at A = 310 nm) and the ps electron pulse (pulse-width 15 ps). The corresponding theoretical f(r) curve for CH2I2 is superimposed on the — 20-ps data set. The changes observed are at r = ca. 2.0 and 3.5 A corresponding to the C—I and I—I internuclear spacings, respectively [reproduced with permission from (96), p. 161],...

Colorado, A. Shen, J.X.X. Vartanian, V.H. Brodbelt, J. Use of infrared multiphoton photodissociation with SWIFT for electrospray ionization and laser desorption applications in a quadrupole ion trap mass spectrometer. Arml. Chem. 1996, 68, 4033-4043. 

The experimental arrangement is similar to that shown in Figure 15.2, but involves the use of a frequency-tripled laser to produce the radiation at A = 121.6 nm (Lymann-a transition) and a second tuneable laser to produce the high ns/nd Rydberg states (i.e. two probe lasers are needed in addition to the photodissociation-pump laser). The resolution achievable with this technique is quite remarkable. 

Table 1. Photodissociation probabilities[20] for C I- CFa+I with laser frequency of co=40323cm . For more details see Refs 18 and 14. RM=R matrix, LD=Log-derivative methods.

FIGURE 9.16 Tandem time-of-flight mass spectrometer, using photodissociation with an excimer laser (Reprinted with permission from reference 9). 

As in threshold photoionization experiments, photofragment ion concentrations are extremely low and it is necessary to utilize signal-enhancement techniques. Although phase-sensitive detection has been used, it is more usual to employ signal-averaged pulse counting. For photodissociation with a pulsed laser, an appropriate gating system synchronized... 

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