Millisecond flash lamps

Electronic flash lamps are used when the light duration has to extend into milliseconds rather than the microsecond range (Ref 7). 

The flash lamps used in archetypal time-resolved techniques produced an intense pulse of short duration and broadband continuous-wave characteristic, and provided information in a millisecond timescale. Gradual improvements, and especially use of laser sources, allow decrease of resolution times to micro-, nano- and femtoseconds [2,10-12],... 

Figure 11.1 shows an example of the schematic diagram of FLA equipment [9]. The flash lamps, e.g., xenon lamps, are energized by discharging a capac-itor/inductor bank flash power supply [10], resulting in millisecond-order radiation onto wafers, to increase the temperature of target material by light... 

Keeping abrupt profiles of dopants is also required to apply the FLA technique to solar cell fabrication process. Figure 11.8 shows secondary ion mass spectroscopy (SIMS) profiles of Cr and P atoms in the bottom layers and of B in the surface doping layer of p-i-n Si stacked films before and after FLA, the structure of which is also schematically shown [34]. The abrupt profiles of the surface B atoms as well as of the bottom Cr and P atoms are maintained after FLA, which results from millisecond-order rapid annealing, and shows the possibility of immediate formation of p-i—n poly-Si structure with only one irradiation of flash lamp for p-i-n stacked a-Si layers. 

The millisecond lower limit of a photo-active lifetime of a sample for pseudo-steady-state experiments is dictated primarily by the maximum rates of repetition of pulses available in most commercial lasers, these being typically of order kHz. Such repetition rates derive from diode lasers, and lasers are more commonly pumped with flash-lamps that operate typically at a repetition rate 10 Hz. 

In transient absorption measurements the number of probing photons in the time window of interest should be sufficiently large to get a satisfactory signal to noise SIN) ratio. For instance, in a nanosecond experiment the number of photons during 1 nsec should preferably be similar to the number of photons during a millisecond experiment. For this reason a continuous lamp can be used as a probe source in the latter experiment, whereas a flash lamp is preferred in the former. 

Short-lived reaction intermediates and products resulting fi-om light flash photolysis have been detected using TOF-MS [76]. A flash lamp was used to induce photochemical reactions of the reactant gases in a reaction vessel. The generated species, such as radicals, could be immediately (in approximately milliseconds) detected by TOF-MS [76]. In other work, a laser beam was combined with an ion cyclotron resonance mass analyzer to follow the process of photodissociation [77]. The dissociation rates and branching ratios for naphthalene ion were measured by means of the time-resolved photodissociation approach. The above-mentioned approaches [76,77] are limited to detection of species generated from gas-phase substrates. 

Eggert [24] showed that the amount of light energy (millisecond exposure time to light of a flash lamp) needed to initiate nitrogen iodide decreased linearly with the initial temperature of the irradiated explosive and converges toward zero at the dark ignition temperature of the sample. Similar observations were obtained with lead azide and some other explosives. These results indicate a photothermal initiation mechanism. Roth [24] arrived at the same conclusion. The... 

The convenience of having a value of Ft(0) as large as possible has been shown in Equations 6.50 and 6.52. A common practice is to pulse a xenon lamp with a steady power of 500 W or 1 kW for periods of several milliseconds when the photomultiplier is used with only five or six dynodes. The lamp is run steadily at a power lower than 500 W or 1 kW but it reaches a much higher power, that is, several times the steady power, when pulsed. Over aperiod of several hundred microseconds, at the peak of the pulse, the intensity of the lamp remains constant. The flash photolysis experiment is timed to happen over this period when the intensity of light is constant at its maximum value. Although the lamp is being pulsed, the probe is a steady light beam for at least several hundred microseconds. 

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