ordinary cw spectra. We attribute this to their direct dependence on the T z j according to eq 1 .We expect that as 2D-ESE techniques are further developed, they will provide a very useful method to enable detailed studies of structure and dynamics in oriented media.The use of absorption and emission spectroscopy of photodissociating molecules is discussed as a means of probing details of dissociation dynamics in extremely short-lived transient species. The study of photodissociation processes has special advantages which derive from the special ability to create well-defined initial conditions and from the simplicity of interpretation of experimental results. Casting the absorption and emission processes in a time-dependent formalism developed by Heller and co-workers enables intuitive connections to be made between spectroscopic features and the underlying dynamics. The absorption spectrum of photodissociating molecules primarily contains information about very short-time dynamics, whereas the emission spectrum reveals more details and encompasses intermediate times as well. Information about both the excited-state and the ground-state potential surfaces is contained in the emission spectrum. Intensities of fundamentals, overtones, and combinations can be used to infer properties of the upper-state surface (such as forces and their gradients). The observed energies and band contours of the same features yield characteristics of the ground-state surface. Experimental results on methyl iodide and ozone are used to illustrate how emission spectra can be used to study reaction dynamics.

We have employed two complementary techniques, molecular beam experiments and ``trajectory surface hopping'' theory, to investigate proton-hydrogen molecule scattering at relative energies between 1 and 7 eV. Absolute cross sections, product translational energy distributions, and product velocity contour diagrams have been obtained from both theory and experiment for the H+ + D2 and D+ + HD isotope arrangements. Agreement is excellent, particularly for a theory which involves no empirical information and no adjustable parameters. By combining results from experiment and theory we have been able to develop a fairly complete, reliable, and simple picture of the dynamics of this elementary collision process. For relative energies below about 3 eV the reaction involves a short-lived collision intermediate, whereas above 4 eV it proceeds by a predominantly direct, impulsive mechanism. Above 5.5 eV momentum transfer is well represented by a hard-sphere collision model involving only short-range repulsive forces. The avoided intersection of the two lowest singlet potential energy surfaces of H3+ is of primary importance in determining the partitioning of energy among products, the likelihood of electronic transitions, the angular and velocity distributions of products, and the competition among various rearrangement, charge transfer, and fragmentation channels.

The effects of collision energy, vibrational mode, and vibrational angular momentum on energy transfer and dissociation in N O 2 + -rare gas collisions: An experimental and trajectory study J. Chem. Phys. 125, 133115 (2006); 10.1063/1.2229207Collision-induced nonadiabatic transitions in the second-tier ion-pair states of iodine molecule: Experimental and theoretical study of the I 2 ( f 0 g + ) collisions with rare gas atomsWe describe a new molecular beam-luminescence method for measuring state-resolved cross sections for the quenching of metastable noble gas atoms, and report values for ''dark'' collisions of Ng*( 3 P 2 , 3 P 0 ) with O 2 (X 3 ⌺ g Ϫ ), where NgϭAr, Kr, and Xe. Cross sections for quenching Q and, in some cases, cross sections for excited products * are also given for a number of state-specific, luminescent monitor reactions. The elastic reaction of Ng* with He or Ne is employed to correct the total disappearance cross section Q T for viewing losses caused by nonquenching processes. The velocity-averaged, quenching cross section Q Q is obtained by subtracting the nonquenching cross section Q N from Q T . Values of Q Q measured at average relative velocity ḡ ͑average relative kinetic energy Ē͒ are deconvoluted to yield Q (ḡ). For Ar* collisions with O 2 , we find Q [ 3 P 2 ] values of 35.6Ϯ1.8 Å 2 for ḡ (Ē) between 690 and 2000 m/s ͑50 and 350 meV͒ that gradually decrease above 2000 m/s, and Q [ 3 P 0 ] values of 46Ϯ4 Å 2 between 690 and 830 m/s ͑50 and 70 meV͒. For Kr* collisions, we report Q [ 3 P 2 ] values of 38.1Ϯ2.5 Å 2 between 575 and 810 m/s ͑46 and 87 meV͒ with no apparent velocity dependence, and a Q [ 3 P 0 ] value of 56Ϯ7 Å 2 at 576 m/s ͑46 meV͒. For Xe* collisions, we find Q [ 3 P 2 ] values of 48Ϯ3 Å 2 at 535 m/s ͑44 meV͒ and 38Ϯ2 Å 2 at 697 m/s ͑73 meV͒, and a Q [ 3 P 0 ] value of ϳ125 Å 2 at 535 m/s ͑44 meV͒. Comparisons with Q values obtained with other techniques that do not require a viewing loss correction are excellent. We also use the Ionic-Intermediate-Curve-Crossing Model ͑IICCM͒ to calculate cross sections for the Ar*( 3 P 2 )ϩO 2 →ArϩO*( 1 D)ϩO( 3 P) reaction. In our application of the model, the product state dissociative continuum is coupled to Ar ϩ O 2 Ϫ through the predissociating O 2 *(E 3 ⌺ u Ϫ ) state that is valence Rydberg in character. Values of Q derived from the model are in good agreement with our experiment.

We have developed a new method for measuring quenching cross sections for metastable atom reactions yielding luminescent products. Application of our method to the Ar*+H2 O system yields an absolute quenching cross section with a strong E−0.5 dependence over the energy range of 0.045–0.130 eV.

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