proach has proven to be particularly promising for following femtosecond chemical reactions in real time. Briefly, a molecular system can be characterized by the electronic potential energy of surfaces on which wave packets propagate.

Experimental efforts in the field of femtochemistry have exploited the pump-probe technique, wherein a pump laser pulse initiates a chemical reaction and a probe laser pulse records a “snapshot” of the chemical reaction at a time controlled by the temporal delay between the pump and probe pulses. By recording snapshots as a function of the temporal delay, one can follow the time evolution of a chemical reaction with time resolution limited only by the duration of the laser pulses. Beyond monitoring the outcome of a normal photoreaction, the phase and frequency of a femtosecond pump pulse can be tailored, as prescribed by theory, to drive a molecular state to a target location on its potential energy surface and then steer it toward a channel that favors a particular photochemical outcome.

For example, say the authors, the excitation pulse might be a femtosecond, linear-chirped laser pulse, which can interact with the wave packet through a so-called intrapulse, pump-dump process. A negatively chirped pulse (frequency components shift in time from blue to red) might be tailored to maintain resonance with the wave packet as it evolves along the excited state surface. In contrast, a positively chirped pulse might quickly go off resonance with the wave packet, and the photoexcitation would be nonselective.

Understanding molecular motions and how they couple to the reaction coordinate is crucial for a comprehensive description of the underlying microscopic processes. This problem is particularly challenging because molecules exhibit strong mutual interactions, and these interactions evolve on the femtosecond time scale because of random thermal motion of the molecules. In essence, understanding the dynamics of a molecular system in the condensed phase boils down to a problem of nonequilibrium statistical physics. Combined with an impressive increase in computational capacity, recent developments in theoretical methodology such as molecular dynamics, path-integral approaches, and kinetic-equation approaches for dissipative systems have enlarged dramatically the scope of what is now theoretically tractable.

Biometric Identification

Personal identification, regardless of method, is ubiquitous in our daily lives. For example, we often have to prove our identity to gain access to a bank account, to enter a protected site, to draw cash from an ATM, to log in to a computer, to claim welfare benefits, to cross national borders, and so on. Conventionally, we identify ourselves and gain access by physically carrying passports, keys, badges, tokens, and access cards, or by remembering passwords, secret codes, and personal identification numbers (PINs). Unfortunately, passports, keys, badges, tokens, and access cards can be lost, duplicated, stolen or forgotten, and pass-



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