Ultra-fast laser spectroscopy is the study of molecules on extremely short time scales (nanoseconds to femtoseconds) after their excitation with a pulsed laser. This method is used extensively to examine the energy states and electron dynamics of any molecule whose reaction to light is of interest. Many different procedures have been developed; some common methods are: ultra-fast transient absorption (TA); time-correlated single photon counting (TCSPC); and time-resolved photo-electron spectroscopy (TRPES). All methods must take into consideration the quantum-mechanical nature of absorption and fluorescence, specifically that individual molecules, even in pure samples, will not emit their photons simultaneously even though they are excited simultaneously, and the rate of decay between the two states is related to the difference in energy between them (see Atomic spectral lines for more information).
This method is used to analyze the relaxation of molecules from an excited state to a lower energy state. Since various molecules in a sample will emit photons at different times following their simultaneous excitation, the decay must be thought of as having a certain rate rather than occurring at a specific time after excitation. By observing how long individual molecules take to emit their photons, and then combining all these data points, an intensity vs. time graph can be generated that displays the exponential decay curve typical to these processes. However, it is difficult to simultaneously monitor multiple molecules. Instead, individual excitation-relaxation events are recorded and then averaged to generate the curve. This is done by splitting a pulsed laser beam into two paths. A pulse along one path travels to a photomultiplier tube (PMT), while another path travels through the sample. The first pulse is detected by a photomultiplier tube, which activates a time-to-amplitude converter (TAC) circuit. This circuit begins to build a charge on a capacitor which will only be discharged once the PMT sends another electrical pulse to the circuit. This electrical pulse comes after the second laser pulse excites the molecule to a higher energy state, and a photon is eventually emitted from a single molecule upon returning to its original state. Thus, the longer a molecule takes to emit a photon, the higher the voltage of the resulting pulse. The central concept of this method is that only a single photon is needed to discharge the capacitor. Thus, this experiment must be repeated many times to gather the full range of delays between excitation and emission of a photon. After each trial, a pre-calibrated computer converts the voltage sent out by the TAC into a time and records the event in a histogram of time since excitation. Since the probability that no molecule will have relaxed decreases with time, a decay curve emerges that can then be analyzed to find out the decay rate of the event. A major complicating factor is that many decay processes involve multiple energy states, and thus multiple rate constants. Though non-linear least squared analysis can usually detect the different rate constants, determining the processes involved is often very difficult and requires the combination of multiple ultra-fast techniques. Even more complicating is the presence of inter-system crossing and other non-radiative processes in a molecule. A limiting factor of this technique is that it is limited to studying energy states that result in fluorescent decay. This method is typical of 'pulse-probe' experiments, where a pulsed laser is used to excite a molecule's electrons from their ground states to higher-energy excited states. A probing light source, typically a xenon arc lamp, is used to obtain an absorption spectrum of the compound at various times following its excitation. As the excited molecules absorb the second pulse, they are further excited to even higher states. After passing through the sample, the light from the arc lamp continues to an avalanche photodiode array, and the data is processed to generate an absorption spectrum of the excited state. Since all the molecules in the sample will not undergo the same dynamics simultaneously, this experiment must be carried out many times, and the data must be averaged in order to generate spectrums with accurate intensities and peaks. Unlike TCSPC, this technique can be carried out on non-fluorescent samples. Time-Resolved Photo-Electron Spectroscopy (TRPES) This method is very similar to Ultra-fast transient absorption, the difference being that the second laser pulse ionizes the molecule. The kinetic energy of the electrons from this process are then detected, through various different methods including energy mapping, time of flight measurements etc. As above, the process is repeated many times, with different time delays between the probe pulse and the pump pulse. This builds up a picture of how the molecule relaxes over time. A variation of this method looks at the positive ions created in this process, and is called Time-Resolved Photo-Ion Spectroscopy (TRPIS) See also * spectroscopy
* Controlling and probing atoms and molecules with ultrafast laser pulses, PhD Thesis
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