Real-time dynamics of photodissociation

In order to model and to analyze time-resolved measurements with finite pulse lengths it is essential to construct the true wavepacket that the laser generates in the excited state and to explore how this wavepacket evolves in time. Chapter 16 closes our survey of molecular photodissociation. On the other hand, it brings us back to Chapter 2, namely to the question of how the photon beam excites the molecule. Section 

1 discusses the excitation process under more general conditions than previously assumed. In Section 16.2 we will review two particular examples of real-time probing of molecular motion, one being representative of direct dissociation and the other one exemplifying indirect or delayed dissociation. 

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We next study how the results of wavepacket dynamics can be tracked experimentally as a real-time history of chemical or physical events. Femtosecond time-resolved spectroscopy enables us to observe nuclear dynamics and to chart the path of chemical reactions in real time, and has been exploited in numerous applications ranging from fundamental studies of real-time motion in the photodissociation of diatomic molecules to stud-... 

Time-resolved ionization offers several advantages as a probe of these wavepackets [41, 42, 343, 360]. For example, the ground state of an ion is often better characterized than higher excited states of the neutral molecule, particularly for polyatomics. Ionization is also universal and hence there are no dark states. Furthermore, ionization provides both ions and photoelectrons and, while ion detection provides mass and kinetic-energy resolution in time-resolved studies [508], photoelectron spectra can provide complementary information on the evolution of the wavepacket [22, 63, 78, 132, 201, 270, 271, 362, 363, 377]. Its utihty for real-time probing of molecular dynamics in the femtosecond regime has been nicely demonstrated in studies of wavepackets on excited states of Na2 [22], on the B state of I2 [132], and on the A state of Nal [201]. Femtosecond photoelectron-photoion coincidence imaging studies of photodissociation dynamics have been reported [107]. 

In the first pump probe experiments with picosecond time resolution the real-time spectra of the Nas E) state (Fig. 4.8) reveal a fast exponential decay caused by ultrafast photo-induced dissociation [306]. This behavior could be well explained with the simple fragmentation model described in Sect. 2.2.2. With femtosecond time resolution it was expected to observe the photodissociation process with more detail in the recorded transients. Especially, the energy dependence of the ultrashort fragmentation process should allow deeper insight into the fragmentation dynamics within a photoexcited molecular beam. 

The general theory for the absorption of light and its extension to photodissociation is outlined in Chapter 2. Chapters 3-5 summarize the basic theoretical tools, namely the time-independent and the time-dependent quantum mechanical theories as well as the classical trajectory picture of photodissociation. The two fundamental types of photofragmentation — direct and indirect photodissociation — will be elucidated in Chapters 6 and 7, and in Chapter 8 I will focus attention on some intermediate cases, which are neither truly direct nor indirect. Chapters 9-11 consider in detail the internal quantum state distributions of the fragment molecules which contain a wealth of information on the dissociation dynamics. Some related and more advanced topics such as the dissociation of van der Waals molecules, dissociation of vibrationally excited molecules, emission during dissociation, and nonadiabatic effects are discussed in Chapters 12-15. Finally, we consider briefly in Chapter 16 the most recent class of experiments, i.e., the photodissociation with laser pulses in the femtosecond range, which allows the study of the evolution of the molecular system in real time. 

Real-time experiments (Khundkar and Zewail, 1990 Zewail, 1991) with a subpicosecond resolution have probed the unimolecular dynamics of NO2 NO + O (Ionov et al., 1993a) and H + CO2 HOCO -> HO 4- CO (Scherer et al., 1987, 1990 Ionov et al., 1993b). The NO2 experiment is described and discussed in section 6.2.3.1 (p. 196). The H + CO2 reaction and ensuing formation of HOCO is initiated by photodissociation of HI in the HI—CO2 van der Waals complex (Fig. 8.8). A subpicosecond laser pulse is used to initiate the reaction while a second laser pulse probes the product formation. The reactants are vibrationally and rotationally cold prior to excitation, and the experiments demonstrate that the H + CO2 reaction proceeds... 

In this book the real-time photodissociation dynamics of small sodium (Nan=3...io) and potassium (Kn=3...9) clusters are studied as a function of cluster size as well as excitation wavelength (Sects. 4.2-4.4). The ratio of dissociative to radiative decay is a measure of the predissociation of an electronic state [122, 133]. For the C state of Naa the electronic predissociation dynamics and especially the localization of its onset are analyzed in detail by ultrafast spectroscopy (Sect. 4.1). 

In this section a rather simple energy-level model is first briefly described, which enables a rough analysis of real-time decay processes obtained by MPI. It has been successfully applied to the investigations of the Nas state. To use the model for the larger alkali clusters, however, it has to be modified. Both models can be regarded as a first approximation to estimate the time constants of the induced photodissociation process. It has to be stated that neither of the two models takes into account the dynamics of wave packets prepared on the repulsive PES. 

In Chap. 3, wave packet propagation could be observed for nearly all of the alkali dimer and trimer systems considered, over a rather long time compared to the wave packet oscillation period. The wave packet dynamics - a fingerprint of the excited molecule - definitely characterize the excited bound electronic state of these molecules. However, with the results on K3 (excited with A 800 nm), another phenomenon, which often governs ultrafast molecular and cluster dynamics, comes into the discussion photodissociation induced by the absorption of single photons. This photoinduced dissociation permits detailed study of molecular dynamics such as breaking of bonds, internal energy transfer, and radiationless transitions. The availability of laser sources with pulses of a few tens of femtoseconds today opens a direct, i.e. real-time, view on this phenomenon. 

The sodium trimer excited to the electronic C state can be regarded as a fascinating model system, which manifests ultrafast predissociation dynamics. While stationary and nanosecond-pump probe spectroscopy gave the first hints that this excited state photodissociates rather fast, real-time TPI spectroscopy opens a window to directly observe these ultrafast processes. But let us first start with a short review of the spectroscopy of this excited electronic state. 

In this book an overview has been given of the amazing opportunities provided by femtosecond real-time spectroscopy applied to small molecules and clusters. Fascinating phenomena such as control of molecular dynamics, selective state preparation, observation of vibrational wave packets on ground state PESs, ultrafast IVR, and photodissociation with unexpected and sometimes exceptional features have been introduced. 

The ultrafast photodissociation dynamics of the Na3 C state was analyzed with time-resolved two-color TPI spectroscopy in the picosecond regime. The two excitation wavelengths required, independently tunable for the pump and the probe pulse, were generated by a home-built synchronization of two mode-locked titanium sapphire lasers. The deconvoluted real-time spectra can be well described by a single exponential decay with a time constant strongly... 

With the advent of femtosecond laser technology [72], it is now possible to study, in real time, the dynamics of elementary chemical reactions. The first example of an application of this technique to a chemical dynamical problem comes from sub-picosecond experiments [73] on the photodissociation of ICN. In the most recent work [73b], a 40-fs pulse (k = 307 nm) excited the ICN molecule to a repulsive state and a delayed, second femtosecond pulse probed the CN fragment (via LIF) as a function of time. By varying the probe wavelength to the red of the free CN bandhead, they were able to observe fluorescence from the CN in the I-CN molecule in the process of falling apart. They found that the transition state lives for about four times the vibrational period of the I-CN bond. In this time interval there is negligible rotation of the parent ICN molecule. 

With Herschbach I worked on photodissociation dynamics and fluorescence. When I finished my Ph.D., Dudley arranged a beginning faculty position for me with A. C. Cope who was then Department Head at MIT. It was a typical old-boy-network arrangement, which would be impossible today. However, I did not stay around and told Cope that I would take the faculty position in another year. I went out to Colorado to the Joint Institute for Laboratory Astrophysics (JILA), at the University of Colorado. I was a postdoc there with Gordon Dunn and Ed Condon. At JILA, I got my real first experience with experiments. I returned to MIT after a year, but I only spent nine months there. In the intervening time. Cope had been removed from being Head, and the MIT chemistry department was squabbling and in disarray. When I went to see the provost, Jerry... 

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