Lasers pulsed

In the previous section we discussed light and matter at equilibrium in a two-level quantum system. For the remainder of this section we will be interested in light and matter which are not at equilibrium. In particular, laser light is completely different from the thennal radiation described at the end of the previous section. In the first place, only one, or a small number of states of the field are occupied, in contrast with the Planck distribution of occupation numbers in thennal radiation. Second, the field state can have a precise phase-, in thennal radiation this phase is assumed to be random. If multiple field states are occupied in a laser they can have a precise phase relationship, something which is achieved in lasers by a teclmique called mode-locking Multiple frequencies with a precise phase relation give rise to laser pulses in time. Nanosecond experiments... 

The FVH representation allows us to visualize the results of more eomplieated laser pulse sequenees. A laser pulse whieh takes f from (0,0,-l) to (0,0,1) is ealled a ii-pulse sinee the f veetor preeesses ii radians about the field veetor. Similarly, a pulse whieh takes f from (0,0,-l) to (+1,0,0) is ealled a Ji/2-pulse. The state represented by tlie veetor (+1,0,0) is a eoherent superposition of tlie upper and lower states of the system. 

Assion A, Baumert T, Bergt M, Brixner T, Kiefer B, Seyfried V, Strehle M and Gerber G 1998 Control of chemical reactions by feedback-optimized phase-shaped femtosecond laser pulses Sc/e/ ce 282 919... 

Apparent non-RRKM dynamics has also been observed in time-resolved femtosecond (fs) experiments in a collision-free enviromnent [117], An experimental study of acetone illustrates this work. Acetone is dissociated to the CH and CH CO (acetyl) radicals by a fs laser pulse. The latter which dissociates by the channel... 

In the present section, we concentrate on coherent preparation by irradiation with a properly chosen laser pulse during a given time interval. The quantum state at time t may be chosen to be the vibrational ground... 

At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

Yan Y X, Gamble E B and Nelson K A 1985 Impulsive stimulated Raman scattering general importance in femtosecond laser pulse interactions with matter, and spectroscopic applications J. Chem. Phys. 83 5391-9... 

SFIG or SFG from a medium that has a strong response in a separate detection anu. By this means, one may fiilly compensate for variations not only in pulse energy, but also in the temporal and spatial substructure of the laser pulses. Some experiments may require measurement of the phase of the nonlinear signal [57]. 

The quantities in this fomuila are defined as in equation Bl.5,32. but with the laser parameters translated into more convenient tenns is the average power at the indicated frequency is the laser pulse duration ... 

Dadap J I, Hu X F, Russell N M, Ekerdt J G, Lowell J Kand Downer M C 1995 Analysis of second-harmonic generation by unamplified, high-repetition-rate, ultrashort laser pulses at Si(OOI) interfaces/ J. Selected Topics Quantum Electron 1 1145-55... 

Time-resolved fluorescence is perhaps the most direct experunent in the ultrafast spectroscopist s palette. Because only one laser pulse interacts with the sample, the mediod is essentially free of the problems with field-matter time orderings that arise in all of the subsequently discussed multipulse methods. The signal... 

The main cost of this enlianced time resolution compared to fluorescence upconversion, however, is the aforementioned problem of time ordering of the photons that arrive from the pump and probe pulses. Wlien the probe pulse either precedes or trails the arrival of the pump pulse by a time interval that is significantly longer than the pulse duration, the action of the probe and pump pulses on the populations resident in the various resonant states is nnambiguous. When the pump and probe pulses temporally overlap in tlie sample, however, all possible time orderings of field-molecule interactions contribute to the response and complicate the interpretation. Double-sided Feymuan diagrams, which provide a pictorial view of the density matrix s time evolution under the action of the laser pulses, can be used to detenuine the various contributions to the sample response [125]. 

Katzenellenbogen N and Grischkowsky D 1991 Efficient generation of 380 fs pulses of THz radiation by ultrafast laser pulse excitation of a biased metal-semiconductor interface Appl. Phys. Lett. 58 222-4... 

Shank C V 1988 Generation of ultrashort optical pulses Ultrashort Laser Pulses and Applications ed W Kaiser (Berlin Springer) pp 5-34... 

Kane D J, Taylor A J, Trebino R and DeLong K W 1994 Single-shot measurement of the intensity and phase of a femtosecond UV laser pulse with frequency-resolved optical gating Opt. Lett. 19 1061-3... 

Dugan M A, Tull J X and Warren W S 1997 High-resolution acousto-optic shaping of unamplified and amplified femtosecond laser pulses J. Opt. Soc. Am. B 14 2348-58... 

Kaiser W (ed) 1988 Ultrashort Laser Pulses and Applications (Berlin Springer)... 

Rulliere C (ed) 998 Femtosecond Laser Pulses (BerWn Springer)... 

Figure B2.3.9. Schematic diagram of an apparatus for laser fluorescence detection of reaction products. The dye laser is syncln-onized to fire a short delay after the excimer laser pulse, which is used to generate one of the reagents photolytically.

The time resolution of these methods is detennined by the time it takes to mitiate the reaction, for example the mixing time in flow tubes or the laser pulse width in flash photolysis, and by the time resolution of the detection. Relatively... 

The experiments were perfonued in a static reaction cell in a large excess of N2 (2-200 bar). An UV laser pulse (193 mu, 20 ns) started the reaction by the photodissociation of N2O to fonu O atoms in the presence of NO. The reaction was monitored via the NO2 absorption at 405 mu using a Hg-Xe high-pressure arc lamp, together with direct time-dependent detection. With a 20-200-fold excess of NO, the fonuation of NO2 followed a pseudo-first-order rate law ... 

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