Laser-pulse molecular dissociation

TIME-RESOLVED PHOTOFRAGMENT FLUORESCENCE AS A PROBE OF LASER-PULSE MOLECULAR DISSOCIATION... 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Infrared femtochemistry Vibrationally induced molecular dissociation and its control employing shaped fs MIR laser pulses... 

Bandrauk s long-term research interests include the dressed-state representation of molecular spectroscopy. His contributions to the nonperturba-tive treatment of molecular spectroscopy from the weak field to strong field limits have been summarized in two chapters in a book he edited in 1993.286 Bandrauk and his coworkers published the first theoretical demonstration of the use of chirped pulses to effect laser bond breaking in less than a picosecond.287 His other firsts include the first prediction of molecular stabilization in intense laser fields288 and the first complete non-Born-Oppenheimer calculation of dissociative ionization of molecules in intense femtosecond laser pulses.289... 

Engel, V., Metiu, H., Almeida, R., Marcus, R.A., and Zewail, A.H. (1988). Molecular state evolution after excitation with an ultra-short laser pulse A quantum analysis of Nal and NaBr dissociation, Chem. Phys. Lett. 152, 1-7. 

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. 

Osorio et al. [134] performed TOF-MS measurements of TNT and RDX on soil surfaces. They used tunable UV radiation from a 130 fs laser to monitor the kinetic energy distribution of N0/N02 photofragments released by the dissociation of TNT and RDX. Analysis of the kinetic energy distribution of the photofragments revealed differences in the processes for NO and NOz ejection in different substrates. Mullen et al. [135] detected triacetone triperoxide (TATP) by laser photoionization. Mass spectra in two time regimes were acquired using nanosecond (5 ns) laser pulses at 266 and 355 nm and femtosecond (130 fs) laser pulses at 795, 500, and 325 nm. The major difference observed between the two time regimes was the detection of the parent molecular ion when femtosecond laser pulses were employed. 

Initial work with respect to Fe(CO)5 photodissociation includes the work mentioned previously by Waller [37] and Seder [31] who studied the state-resolved photochemical breakdown. A number of smdies have been reported which looked at the ultrafast (i.e., sub-picosecond) photodissociation dynamics of Fe(CO)5. Examples of such smdies include that of Banares et al. [57], which looked at the photodissocation dynamics of Fe(CO)5 in a molecular beam using femtosecond laser pulses via two-photon pumping at 400 nm followed by non-resonant ionisation at 800 nm. Detection of the photoproducts was by means of a time-of-flight mass-spectrometer. The timescale for the dissociation of the CO ligands was measured, and it was found that Fe(CO)4 was formed after 20 5 fs, Fe(CO) formed after 100 fs, and complete dissociation of the metal and all ligands sometime after 230 fs. 

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