State preparation electronic excitation

The theory is only in partial agreement with experiment,but both indicate that in the case of autoionization of a single state by electronic excitation one may usually expect a distribution among both vibrational and electronic states of the resulting ion rather than a nearly pure preparation in any specific state. 

There exist a series of beautiful spectroscopy experiments that have been carried out over a number of years in the Lineberger (1), Brauman (2), and Beauchamp (3) laboratories in which electronically stable negative molecular ions prepared in excited vibrational-rotational states are observed to eject their extra electron. For the anions considered in those experiments, it is unlikely that the anion and neutral-molecule potential energy surfaces undergo crossings at geometries accessed by their vibrational motions in these experiments, so it is believed that the mechanism of electron ejection must involve vibration-rotation... 

These results show the functional ability of GA to act as quencher of electronically excited states in food systems either as non-processed gum or spray-drying microencapsulated preparations. 

Figure 37. Electronic excitation of the NaK wavepacket from the inner turning point of the ground X state. The X A transition is considered. The initial wave packet is prepared by two quadratically chirped pulses within the pump-dump mechanism. Taken from Ref. [37].

In this book we shall write the Hamiltonian as an (algebraic) operator using the appropriate Lie algebra. We intend to illustrate by many applications what we mean by this cryptic statement. It is important to emphasize that one way to represent such a Hamiltonian is as a matrix. In this connection we draw attention to one important area of spectroscopy, that of electronically excited states of larger molecules,4 which is traditionally discussed in terms of matrix Hamiltonians, the simplest of which is the so-called picket fence model (Bixon and Jortner, 1968). A central issue in this area of spectroscopy is the time evolution of an initially prepared nonstationary state. We defer a detailed discussion of such topics to a subsequent volume, which deals with the algebraic approach to dynamics. 

In addition to providing a novel approach to the preparation of chiral compounds, this type of chemistry may allow one to inquire into the subtle stereochemical details of some crystal-state reactions. For example, what are the approach geometry and the preferred side of attack in the addition of bromine to a chiral olefin (259) What can be learned of the geometry of the labile electronically excited species involved in (2 + 2) photocycloaddition reactions (260) ... 

A major technological innovation that opens up the possibility of novel experiments is the availability of reliable solid state (e.g., TiSapphire) lasers which provide ultra short pulses over much of the spectral range which is of chemical interest. [6] This brings about the practical possibility of exciting molecules in a time interval which is short compared to a vibrational period. The result is the creation of an electronically excited molecule where the nuclei are confined to the, typically quite localized, Franck-Condon region. Such a state is non-stationary and will evolve in time. This is unlike the more familiar continuous-wave (cw) excitation, which creates a stationary but delocalized state. The time evolution of a state prepared by ultra fast excitation can be experimentally demonstrated, [5,7,16] and Fig. 12.2 shows the prin-... 

Eq. (21), which directly reflects the non-Born-Oppenheimer dynamics of the system. Assuming that the system is initially prepared at x = 3 in the diabatic state /2), the corresponding initial distribution pg mainly overlaps with orbits A and C, since at x = 3 these orbits do occupy the state /2). Similarly, excitation of /i) mainly overlaps with orbits of type B, which at x = 3 occupy the state /i) (see Fig. 32). In a first approximation, the electronic population probability P t) may therefore be calculated by including these orbits in the... 

VMP, a term coined by Crim and coworkers in 1987 [26], has been applied extensively by him and others since then [27-33], However, some VMP studies were carried out even earlier (see Section 2.3). As mentioned earlier, direct photodissociation, Figure 2.1a, involves electronic excitation of vibrationless ground state parent molecules to an excited PES, while in VMP, Figure 2.1b, excitation of molecules initially prepared in a particular vibrational state is involved. Therefore, in VMP, two- or three-color photons are required to induce a double resonant... 

The intensity of the absorption leading to VMP, S , depends on the transition probabilities for excitation from the ground vibrational state to the prepared intermediate state and for promotion of molecules from this state to the excited electronic state. 

As mentioned above, the H photofragment yield depends on the SRS vibrational excitation probability, the FC overlap between the vibrational wavefunc-tions in the ground and excited electronic states, the electronic transition dipole moment and on the photodissociation channel. Since in the VMP process the combined SRS-I-UV excitation energies are in the -43,900-44,530 cm i range, the CH3NH2 A state is accessed from all the initially prepared vibrational states. It was therefore concluded [34, 81, 82] that the main player determining the H yield... 

All the methods used to evaporate metals for atom synthesis were developed originally for the deposition of thin metal films. The more important of these techniques are shown schematically in Fig. la-d. Most of the evaporation devices can be scaled to give amounts of metal ranging from a few milligrams per hour for spectroscopic studies to 1-50 gm/hour for preparative synthetic purposes. Evaporation of metals from heated crucibles, boats, or wires (Fig. la-c) generally gives metal atoms in their ground electronic state. Electronic excitation of atoms is possible when metals are vaporized from arcs, by electron bombardment, or with a laser beam (Fig. Id). The lifetime of the excited states of... 

There can be a difference between the dissociation of polyatomic molecules and delayed ionization in the nature of the initial excitation. In ZEKE spectroscopy the state that is optically accessed (typically via an intermediate resonantly excited state) is a high Rydberg state, that is a state where most of the available energy is electronic excitation. Such a state is typically directly coupled to the continuum and can promptly ionize, unlike the typical preparation process in a unimolecular dissociation where the state initially accessed does not have much of its energy already along the reaction coordinate. It is quite possible however to observe delayed ionization in molecules that have acquired their energy by other means so that the difference, while certainly important is not one of principle. 

Recent advances in experimental techniques, particularly photoionization methods, have made it relatively easy to prepare reactant ions in well-defined states of internal excitation (electronic, vibrational, and even rotational). This has made possible extensive studies of the effects of internal energy on the cross sections of ion-neutral interactions, which have contributed significantly to our understanding of the general areas of reaction kinetics and dynamics. Other important theoretical implications derive from investigations of the role of internally excited states in ion-neutral processes, such as the effect of electronically excited states in nonadiabatic transitions between two potential-energy surfaces for the simplest ion-molecule interaction, H+(H2,H)H2+, which has been discussed by Preston and Tully.2 This role has no counterpart in analogous neutral-neutral interactions. 

Asymptotically, one beam can be prepared to carry electronically excited atoms (A) and the other ground-state atoms (B), and thus the wave function can be written as... 

The photofragmentation that occurs as a consequence of absorption of a photon is frequently viewed as a "half-collision" process (16)- The photon absorption prepares the molecule in assorted rovibrational states of an excited electronic pes and is followed by the half-collision event in which translational, vibrational, and rotational energy transfer may occur. It is the prediction of the corresponding product energy distributions and their correlation to features of the excited pes that is a major goal of theoretical efforts. In this section we summarize some of the quantum dynamical approaches that have been developed for polyatomic photodissociation. For ease of presentation we limit consideration to triatomic molecules and, further, follow in part the presentation of Heather and Light (17). 

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