Femtosecond pump-probe laser excitation

Ultrafast molecular elimination of iodine from IF2C-CF2I has been studied using the velocity map ion imaging technique in combination with femtosecond pump-probe laser excitation.51 By varying the femtosecond delay between pump and probe pulse, it has been found that elimination of molecular iodine is a concerted process, although the two carbon-iodine bonds are not broken synchronously. 

Fig. 16.1. (a) Schematic illustration of a femtosecond pump-probe experiment. The pump laser with wavelength Ai excites the molecule from the ground-state potential Vo to an excited state-potential V. After a delay r the probe laser with wavelength A2 excites the transient molecule to a second excited-state potential V2. (b) Absorption signal of the transient molecule if the wavelength of the probe laser is tuned to the asymptotic wavelength Ag0 (upper part) or to a wavelength shifted to the red of Ag0 (lower part). Reproduced from Zewail (1988). 

The spectroscopic tool to be considered here is femtosecond pump/probe spectroscopy. This experimental technique uses two ultrashort laser pulses which are time-delayed with respect to each other. They are sent into a molecular sample and a signal is recorded as a function of the delay-time between the pulses. To be more specific, we assume the molecule to be in an inital state 0o) O). Here o) denotes the wave function for the nuclear motion and 0) the wave function of the electrons (the adiabatic separation of nuclear and electronic motion is assumed throughout). The pump pulse induces a transition and the resulting wave function which describes the molecule after the interaction with the electric field may be assigned as 0i l). We treat electronic excitation so that the molecule is prepared in another electronic state 1). After the pump pulse passed the sample, the molecule evolves unperturbed until the probe pulse starts interacting. This interaction results in a second excitation to (in our case) a final electronic state 2) with the respective nuclear wave function 1 2) The scheme just described is depicted in Figure 1 and illustrates the idea of many pump/probe experiments. 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

Figure 1.4. Experimental and theoretical femtosecond spectroscopy of IBr dissociation. Experimental ionisation signals as a function of pump-probe time delay for different pump wavelengths given in (a) and (b) show how the time required for decay of the initally excited molecule varies dramatically according to the initial vibrational energy that is deposited in the molecule by the pump laser. The calculated ionisation trace shown in (c) mimics the experimental result shown in (b).

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

TRPES has been recently reviewed and details of the experimental method and its interpretation can be found elsewhere [5], Trans-azobenzene was introduced via a helium supersonic molecular beam into the interaction region of a magnetic bottle photoelectron spectrometer. The molecules were photoexcited by a tunable femtosecond laser pulse (pump pulse) with a wavelength of 280-350nm. After a variable time delay, the excited molecules were ionized by a second femtosecond laser pulse (probe pulse) with a wavelength of 200 or 207nm. The emitted photoelectrons were collected as a function of pump-probe time delay and electron kinetic energy. 

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