Infrared lasers, time-resolved measurements using

Time-Resolved Measurements Using an Infrared Laser... 

In laser measurements on hydrogen halides, the system under investigation is itself all, or part, of the active laser medium. Similar experiments have not yet been made on other molecules. However, in related experiments on CO, in the author s laboratory [242,432] and elsewhere [433], a CO cw laser has been used to probe the vibrational distributions in a reaction system outside the optical cavity that may act as an amplifier or absorber of lines from the laser. Time-resolved observations can be made for as long as one chooses after the reaction is initiated. If tunable infrared lasers become readily available this technique is likely to be applied more widely. 

A very widely employed method for the measurement of spin-orbit state-specific rate constants is the time-resolved measurement of the concentrations of individual atomic levels after formation of these species from a suitable precursor, either by flash photolysis [13], or, more recently, by laser photodissociation. The concentrations of the various atomic reactant states are monitored by atomic absorption or fluorescence spectroscopy using atomic emission sources [14], or, for spin-orbit-excited states, by observation of the spontaneous infrared emission [15-18]. Recently, Leone and co-workers have utilized gain/absoiption of a colour centre and diode infrared laser to probe the relative populations of ground and spin-orbit excited halogen atoms produced in a chemical reaction [19] and also by photodissociation [20],... 

In this chapter, millisecond time-resolved infrared measurements are first described in Section 20.2 for this time scale, time resolution is set by the time needed to measure (scan) a spectrum. Then, microsecond to nanosecond time-resolved measurements, which are limited by the detector response time are described in Section 20.3, and finally, picosecond to femtosecond time-resolved measurements, the time resolution for which is determined by the width of the laser pulse used for the measurement, are described in Section 20.4. 

For microsecond to nanosecond time-resolved infrared absorption measurements, three types of spectroscopic methods have been developed (i) a method using an infrared laser, (ii) a method using a dispersive spectrometer, and (iii) a method using an FT-IR spectrometer. The time resolution of each of these is limited to the fastest time-response capability of the detector used. 

Difference-frequency laser (cw) (see Section 20.4.2.1 for the generation of difference frequency) By using various nonlinear crystals, this laser can cover almost the entire mid-infrared region (the lower limit is about 550 cm ). Since the width of this laser line is very narrow, this laser is important for performing high-resolution time-resolved measurements in the gaseous state. 

Related experiments have recently been carried out in Xe (sc) and Kr (sc) with a time-resolved infrared (TRIR) spectrometer . In this system, reaction is initiated with a laser as before, but spectra are measured one frequency at a time with a continuous diode laser as the IR source. This apparatus, which has a time resolution of ca. 10 s, has been used to observe the complete set of M(CO)5Ng complexes (M = Cr, Mo, W Ng = Kr, Xe) and has provided tentative evidence for W(CO)sAr in Ar (sc). The measurements are carried out with a small pressure of added CO chosen such that the complexes have lifetimes from 100 ns to 2 qs. The rate constants for reaction with CO increase as follows Xe < Kr < Ar W < Mo Cr. The IR spectra are supplemented by UV/visible spectra of Cr(CO)sNg, which are in satisfactory agreement with matrix spectra. 

Rate constants for reaction of the CH radical with a number of atomic and molecular collision partners have been reported, with multiple-photon dissociation of suitable precursor molecules using either infrared or ultraviolet " laser radiation used as the pulsed photolysis source, and laser-induced fluorescence near 431 nm employed as a sensitive time-resolved detection method. A similar technique has been used to measure removal rates of CH2 and CDj with... 

Time-resolved infrared spectroscopy (TRIR) has been outstandingly successful in identifying reactive intermediates and excited states of both metal carbonyl [68,69] and organic complexes in solution [70-72]. Some time ago, the potential of TRIR for the elucidation of photochemical reactions in SCFs was demonstrated [73]. TRIR is particularly suited to probe metal carbonyl reactions in SCFs because v(CO) IR bands are relatively narrow so that several different species can be easily detected. Until now, TRIR measurements have largely been performed using tunable IR lasers as the IR source and this has restricted the application of TRIR to the specialist laboratory [68]. However, recent developments in step-scan FTIR spectroscopy promise to open up TRIR to the wider scientific community [74]. 

Transition radiation is considerably weaker than Cerenkov radiation, however since it is a surface phenomenon it avoids problems with radiator thickness and reflections inherent to Cerenkov-generating silica plates. Optical TR can be measured using a streak camera. An optical TR system has been used to time-resolve the energy spread of an electron macropulse in a free-electron laser facility [10]. Interferometry of coherent, far-infrared TR has been used to measure picosecond electron pulse widths and detect satellite pulses at the UCLA Satumus photoinjector, using charges on the order of 100 pC [11],... 

The detection of short-lived transient species is often achieved by flash photolysis where an extremely short flash of UV/Vis radiation from a laser generates a high concentration of transient species, and a second probe beam monitors any changes that occur after the flash. Traditionally, UVA is spectroscopy has been used as a detection method. However, time-resolved infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR detection, also has a long history. There are several different approaches to fast IR spectroscopy and the method of choice depends upon the timescale of the reaction. Measurements on the nanosecond to millisecond timescale are obtained using point-by-point techniques or by step-scan FTIR. In the point-by-point approach, a continuous wave IR laser (GO or diode) or globar is used as the IR source, which is tuned to one particular IR frequency (Figure 3). ... 

In this book, most chapters deal with FT-IR spectrometry and its applications to various methods of infrared spectroscopic measurements. Only terahertz spectrometry in Chapter 19 and a large part of time-resolved infrared spectrometry in Chapter 20 are laser-based measurements. This shows how widely FT spectrometry is used at present in the measurements of vibrational spectfa. 

Picosecond to femtosecond time-resolved infrared absorption measurements were initiated in the middle of the 1980s. In 1984, Heilweil et al. [17] studied the dynamics of vibrational relaxation by using picosecond infrared pulses obtained from an OPA (LiNb03) excited by a mode-locked Nd YAG laser. 

In 2005, Towrie et al. [30] developed another time-resolved infrared spectrometer capable of performing femtosecond to microsecond time-resolution measurements, by adding to their spectrometer described in Ref [29] a sub-nanosecond Q-switch Nd YV04 laser (wavelength 1064 nm, pulse width 0.6 ns). The pulses generated by this laser were electronically synchronized with the probe pulses with about 0.3 ns jitter, and the harmonics of pulses from this laser were used as the pump pulses. 

In practice, an electromagnetic pulse with an infinitely short width does not exist, but ultrashort laser pulses are now used for various spectroscopic measurements. Terahertz spectrometry described in Chapter 19 is based on femtosecond laser pulses. In Chapter 20, time-resolved infrared spectroscopic methods using picosecond to femtosecond laser pulses are described. Such ultrashort laser pulses have large spectral widths in the frequency domain. Let us discuss the relation between the pulse width in the time domain and its spectral width expressed in either frequency or wavenumber. 

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