Lutz Hüwel

Single collision ion–molecule reactions at thermal energy: Rotational and vibrational distributions from N++CO→N+CO+

A new apparatus is developed and used to obtain nascent vibrational and rotational distributions in the ground electronic state of CO+ formed from the charge transfer reaction N+(3P)+CO (Xu20091Σ+)→N(4S)+CO+ (Xu20092Σ+,v,J)+0.52 eV, at approximately thermal energies. The device utilizes a flow tube for the production of thermal N+ ions in a helium buffer and a large diameter sampling orifice which delivers the ions via a mild free jet expansion into a low pressure chamber. The expansion is crossed by a stream of reactant CO molecules and the CO+ product states are probed by laser‐induced fluorescence. Although the energy available is sufficient to populate CO+ vibrational states up to v″=2, the major vibrational channel in the CO+ product is v″=0. The relative vibrational distribution is found to be: Nv=0≳0.81 (observed under single collision conditions), Nv=1<0.15 (not observed), and Nv=2≊0.04 (observed only under nonsingle collision conditions). The rotational distribution in the v″=0 state is characterized clo...

Laser‐induced fluorescence measurement of nascent vibrational and rotational product state distributions in the charge transfer of Ar++N2→Ar+N+2 (v=0,1) at 0.2 eV

A novel experimental technique couples a flowing afterglow ion source with a supersonic nozzle expansion in order to deliver high densities of relatively low kinetic energy ions into a low pressure chamber. The technique is used to study the charge transfer reaction Ar+(2P3/2)+N2(v=0) →Ar(1S0)+N+2 (v+0,1;N)+ΔE=0.18 eV under single collision conditions at 0.24 eV c.m. Nascent rotational and vibrational state distributions are obtained by the method of saturated laser‐induced fluorescence probing. It is found that a substantial fraction of the available energy is partitioned into internal excitation of the N+2 product molecule. The higher of the two energetically accessible vibrational levels N+2 (v=1) is populated in 89±9% of the charge transfer collisions. The nascent rotational distribution in N+2 (v=1) is characterized by a Boltzmann distribution with Trot=700±50 K. The results are compared with a number of recent state‐selected experiments on charge exchange in ArN+2 , ArH+2 , and NCO+ systems. It is s...

Laser-generated spark morphology and temperature records from emission and Rayleigh scattering studies

Using both Rayleigh scattering and time-resolved emission spectroscopy, we have recorded the spatial and temporal evolution of laser-generated sparks in argon from changes during the first ten of nano-seconds to complete dissipation, which occurs in a time span of approximately 5 ms. Maps of either emission intensity or argon density spanning the entire region affected by the energy deposited by the laser show the dissipation of the spark in detail. Immediately after ignition, the argon plasma occupies an ellipsoidal volume of roughly 3-mm vertical (axial) length. After approximately 20-40 micros, the spark region has transformed into a toroidal shape in a plane perpendicular to the vertical axis, with a radius of approximately 1.5 mm. The torus rises slowly up and expands noticeably in the radial direction. A record of peak temperatures of the spark ranging from approximately 10,000 K at 60-micros delay time to approximately 450 K at 4-ms delay time indicate cooling rates from approximately 100 to 1 K/micros at these times.

Vibrationally resolved lifetimes of the 21Σu+ state of Na2

Lifetimes of partially resolved ro-vibrational levels of the Na2 21Σu+ double well state have been measured for the first time. Ground state sodium dimer molecules in a molecular beam are resonantly excited by the doubled output of a 10 ns pulsed dye laser in the range 333-357 nm. After being allowed to decay for a predetermined time interval, the surviving excited molecules are ionized by 532 nm photons from a delayed Nd:YAG laser and detected in a linear time-of-flight mass spectrometer. By appropriate tuning of the excitation laser and systematic variation of the probe laser delay, lifetimes are obtained for vibrational levels in the range from 22 to 57. At zero rotation, the three lowest vibrational quantum numbers that we have explored (22, 25, and 28) correspond to wavefunctions whose probability densities are appreciable only in the inner well. Levels with larger quantum numbers are located above the barrier, which, for the rotation-free case, lies between quantum numbers 33 and 34. Because of the congested nature of the excitation spectrum and the experimental resolution of about 0.2 cm-1 available to us, our experimental results are only partially quantum state resolved. Nevertheless, we can discern a decrease in lifetime from about 50 to 40 ns for the inner well levels and a slight increase in lifetime with increasing quantum number for levels above the potential barrier. We have also performed lifetime calculations based on the LEVEL and BCONT programs made available by Le Roy, the latter of which was modified by McGeehan. When limited to bound-bound transitions, theoretical lifetimes for levels above the barrier are systematically larger than experimental values by a factor of almost two. With the addition of bound-free transitions, agreement between experiment and theory is, for the most part, within the experimental uncertainties.