We report the spontaneous formation of a plasma from a gas of cold Rydberg molecules. Double-resonant laser excitation promotes nitric oxide, cooled to 1 K in a seeded supersonic molecular beam, to single Rydberg states extending as deep as 80 cm;{-1} below the lowest ionization threshold. The density of excited molecules in the illuminated volume approaches 1x10;{13} cm;{-3}. This population evolves to produce free electrons and a durable cold plasma of electrons and intact NO+ ions.

Spontaneous avalanche to plasma splits the core of an ellipsoidal Rydberg gas of nitric oxide. Ambipolar expansion first quenches the electron temperature of this core plasma. Then, long-range, resonant charge transfer from ballistic ions to frozen Rydberg molecules in the wings of the ellipsoid quenches the centre-of-mass ion/Rydberg molecule velocity distribution. This sequence of steps gives rise to a remarkable mechanics of self-assembly, in which the kinetic energy of initially formed hot electrons and ions drives an observed separation of plasma volumes. These dynamics adiabatically sequester energy in a reservoir of mass transport, starting a process that anneals separating volumes to form an apparent glass of strongly coupled ions and electrons. Short-time electron spectroscopy provides experimental evidence for complete ionization. The long lifetime of this system, particularly its stability with respect to recombination and neutral dissociation, suggests that this transformation affords a robust state of arrested relaxation, far from thermal equilibrium.

We investigate the origin of the observed fission of the stronger S–H bond over the weaker C–S bond in CH3SH excited at 193 nm using the complementary techniques of mass-resolved photofragment time-of-flight spectroscopy and emission spectroscopy. The velocities and angular distributions of the CH3S and SH photofragments show that both C–S and S–H bond fission occur on a subpicosecond timescale and impart considerable energy to relative product translation. The dispersed emission from photoexcited CH3SH molecules in a flow cell evidences a progression in the CH3 umbrella mode and combination bands with one quantum in the C–S stretch, but no progression with S–H stretch. Examination of the results with reference to previous ab initio calculations of the excited state surfaces reveals the importance of nonadiabatic coupling in the dissociation dynamics. This is a clear example of selective bond fission upon excitation of an electronic state that is not repulsive in the bond that breaks. We discuss the implication of the work with respect to using the Born–Oppenheimer approximation in reactive collisions near a saddle point along the reaction coordinate.

Angular distributions are reported for energy-resolved electrons obtained by resonance-enhanced multiphoton ionization of alkaline-earth atoms through excited intermediate valence states. The angular distributions are measured as functions of the independently variable polarizations of the exciting and ionizing laser beams. This report treats the following states, resonantly formed and photoionized: Ca(4s4p) 'Pl, Sr(5s5p) 'Pi, Sr(5s5p) Pl, Ba{6s6p) 'Pl, and Ba{6s6p) Pl. Systematic behavior in the resulting photoelectron angular distributions is explored. The effects of the hyperfine interaction are apparent.

CH3SH ultraviolet absorption cross sections in the region 192.5-309.5 nm and photodecomposition at 222 and 193 nm and 296 K A crossed lasermolecular beam study of the one and two photon dissociation dynamics of ferrocene at 193 and 248 nm These experiments use molecular photodissociation of CH 3 SH to probe the dynamics and the influence of nonadiabatic coupling in the transition state region of the CH 3 +SH--+CH 3 S+H reaction. Photoexcitation at 222 and 248 nm in the first of two absorption bands accesses the lower of the two coupled potential energy surfaces near the saddle point of the excited state reaction coordinate. Measurement of the resulting photofragments' velocities and angular distributions determine the branching between the CH 3 +SH and the CH 3 S+H exit channels. At all wavelengths within the first absorption band, we observe preferential fission of the stronger S-H bond over the weaker C-S bond. Fission of the C-S bond occurs only to a small degree at 222 nm and is not observable at 248 nm. Comparison with our earlier data at 193 nm, corresponding to excitation to the upper bound adiabat which is nonadiabatically coupled to the lower dissociative surface reached at 222 nm, shows that the branching ratio between C-S bond fission and S-H bond fission is a factor of eight larger at 193 nm. To probe the forces in the Franck-Condon region, we also measure the photoemission spectrum from dissociating CH 3 SH excited at 222 nm and compare it to the previous measurement at 193 nm. The 222 nm spectrum evidences emission into the S-H stretch and methyl stretch vibrations but not into C-S stretching modes, consistent with the dominance of S-H fission on the lower adiabat, while the 193 nm emission spectrum, reassigned here, has only a progression in the C-S stretch. The comparison of the spectra suggests a model in which stretching along the C-S coordinate on the bound upper state occurs as the amplitude couples nonadiabatically to the lower dissociative surface, allowing the molecule to access the region near the saddle point on the lower surface at extended C-S bond lengths. This results in better overlap with the C-S fission exit channel and thus an increased branching to C-S bond fission over that observed upon direct excitation to the lower dissociative surface at 222 nm. To further advance the experimental conclusions, we present collaborative calculations of the potential energy surfaces using the effective valence-shell Hamiltonian method developed by Freed and co-workers.