Threshold photoelectron spectra (ZEKE) of methyl iodide, bromide, and chloride are investigated using a one-color, two-photon ionization. Resonances due to a dissociative intermediate state are responsible for differences between spectra of the three halides, with long progressions in the C–I stretching vibration being observed in CH3I, where the one photon absorption is near the maximum of the dissociative continuum, while the vibrational structure is much less prominent in the lighter halides, where this does not occur. The spectra provide accurate ionization potentials of the three compounds as well as insights into their dissociation dynamics, and extensive information about the vibrational structure of their cations.

In this manuscript the available experimental and theoretical information on (NO)2 and its ions is summarized and reviewed, and high resolution two photon, one color photoelectron spectra of the dimer are presented. The new spectra yield a wealth of information about the dimer cation, including possibly all six of its vibrational frequencies as well as several anharmonic intermode constants. The most consistent interpretation of the contradictory data available in the literature and of the present results is to assign the gas phase photoelectron spectrum to cis–cis ionization, and the matrix spectra to the trans (NO)2+ ionic ground state. The similarity between the observed matrix frequencies with the gas phase values reflects the insensitivity of the NO stretching fundamentals of the dimer cation to conformation, a point of view supported by the results of theoretical calculations. Despite the problems of theory in computing properties of the (NO)2 species, the experimental structures and molecular constants appear to be in an acceptable agreement with the most recent computational results.

Threshold photoionization spectroscopy [ zero kinetic energy (ZEKE), pulsed Ðeld ionization (PFI), mass analysed threshold ionisation (MATI) ] usually takes advantage of multiphoton excitation through bound intermediate states, but we show that excellent quality, high-resolution spectra can also be obtained in one-color experiments using two-and three-photon ionization. An enhancement similar to resonance Raman yields changed spectra if the excitation proceeds via continuous intermediate states. A molecule which after absorbing a photon started to dissociate is stabilized on the ionic surface by a delayed second photon, with the observed spectra reÑecting geometry changes due to this arrested dissociation process. Using two colors, the technique should be applicable to almost any molecule, and yield a detailed insight into its dissociation dynamics.

The lifetimes of high Rydberg states of the methyl halides CH31, CH3Br and CH3Cl have been studied. A pulsed dye laser tuned near half the ionization energy was used, with preprogrammed field pulses, and the high Rydberg states of the compounds in selected ranges of n values were ionized after a variable delay. Plots of the electron signal versus delay yielded information about the Rydberg state lifetime. The clearest trend was found for around n = 12&160: the lifetimes of the states decreased by more than an order of magnitude from CH31 (7 w 12.3 ps) through CH3Br to CH3Cl. Within the range studied, the theoretically predicted increase in lifetimes (n' or n3) was not found, and in fact the highest group of states studied (with n > 215) exhibited for all three compounds relatively short lifetimes (w2-3 ps), possibly due to collisional processes. IntroductionPhotoelectron spectroscopy has the advantage of being in principle applicable to any molecule, since every molecule contains electrons. When a molecule is ionized by a photon, most of the excess energy is carried away as kinetic energy of the free electron. The differences between the energies of the ionizing photons and those of the released electrons yield information about the energy differences between the quantum states of the neutral parent molecules and those of the resulting ions. The difficulty lies in determining the exact electron energies. While focusing photons by lenses or mirrors, and directing them onto a detector does not alter their energy, a similar treatment of electrons with electromagnetic mirrors or lenses does, and it is this fact that limits the resolution of conventional photoelectron spectroscopy. A quantum leap in overcoming this difficulty represented the use of tunable laser sources, and the so-called threshold photoelectron spectroscopy [14]: in principle, the realization that the problem can be avoided if one selectively detects only those ionization events that leave the electrons with no or very little kinetic energy. In the original ZEKE idea [3, 41 the molecule was ionized by a tunable, narrow bandwidth laser source, and the zero kinetic energy electrons produced when the photons have exactly the right energy were extracted by an electric field and detected. To discriminate against any electrons formed with nonzero kinetic energy, the extracting field pulse is delayed with respect

The self-energy and the density of states for holes in degenerate p-type silicon are calculated taking into account the interaction with the local mode of the boron acceptor and one and two optical phonons. The current-voltage characteristics of a MIS tunnel junction is obtained using the transfer Hamiltonian model. Adjusting parameters in d21/dU2 to experimental data the line width of the optical phonon and the effective two phonon deformation potential is obtained. I n contrast to other authors, the calculations demonstrate that even a small but energy independent collision time of the carriers has a minor influence on the structures in d21/dU2. The interaction of holes with the local mode is essentially due t o silicon atoms vibrating in the iieighbourhood of the boron ion.Es wird die Selbstenergie und die Zustandsdichte fur Liicher in entartetem p-Silizium mit Beriicksichtigung der Wechselwirkung mit den lokalisierten Akzeptorschwingungen und ein bzw. zwei Phononen numerisch berechnet. Durch Anpassen der iiber das Transfer-Hamiltonian-Model1 errartenen 62r/d V-Kennlinie eines MIS-Tunnel-Kontakts an experimentelle Daten wird die Linienbreite des optischen Phonons und das effektive Zwei-Phonon-Deformationspotential erhalten. Im Gegensatz zu der Meinung anderer Autoren zeigen die Rechnungen, daB die Stoozeit der Ladungstriiger nur einen geringen EinfluS auf die Struktnren in d21/dU2 hat und, daB die Wechselwirkung der Locher mit den lokalisierten Bor-Schwingungen hauptsachlich iiber benachbarte Siliziumatome erfolgt.