RESUMEN
Adiabatic alignment of CH(3)I, induced by the anisotropic interaction of this symmetric top molecule with the intense field of a nonresonant infrared laser pulse, has been studied using velocity map imaging. We are using photodissociation imaging with pulsed nanosecond lasers to probe the distribution of the molecular axis in the laboratory space. In contrast to the commonly used probing with femtosecond laser pulses, this technique directly yields the degree of alignment over an extended space-time volume. This will be relevant for future reactive scattering experiments with laser-aligned molecules. The obtained degree of alignment, (cos (2)θ), measured as a function of the infrared laser intensity, agrees well with a quantum calculation for rotationally cold methyl iodide. The strong infrared laser is also found to modify the photofragmentation dynamics and open up pathways to CH(3)I(+) formation and subsequent fragmentation.
RESUMEN
Absolute total photodetachment cross sections of O(-) and OH(-) anions stored in a multipole radio frequency trap have been measured using a novel laser depletion tomography method. For OH(-) the total cross sections of 8.5(1)(stat)(3)(syst) and 8.1(1)(stat)(7)(syst)x10(-18) cm(2), measured at 662 and 632 nm, respectively, were found constant in the temperature range of 8-300 K. The O(-) cross sections 5.9(1)(stat)(2)(syst) and 6.3(1)(stat)(2)(syst)x10(-18) cm(2) measured at 170 K at 662 and 532 nm, respectively, agree within error estimations with preceding experiments and increase the accuracy of the widely used calibration standard for relative photodetachment measurements of diverse atomic and molecular species.
RESUMEN
The H3+ ion and its deuterated isotopologues H2D+, D2H+ and D3+ play an important role in astrophysical and laboratory plasmas. The main challenge for understanding these ions and their interaction at low temperatures are state-specific experiments. This requires manipulation and a simple but efficient in situ characterization of their low-lying rotational states. In this contribution we report measurements of near infrared (NIR) absorption spectra. Required high sensitivity is achieved by combining liquid nitrogen cooled plasma with the technique of NIR cavity ringdown absorption spectroscopy. The measured transition frequencies are then used for exciting cold ions stored in a low-temperature 22-pole radiofrequency ion trap. Absorption of a photon by the stored ion is detected by using the laser-induced reactions technique. As a monitor reaction, the endothermic proton (or deuteron) transfer to Ar is used in our studies. Since the formed ArH+ (or ArD+) ions are detected with near unit efficiency, the stored ions can be characterized very efficiently, even if there are just a few of them.
RESUMEN
This contribution summarizes a variety of results and ongoing activities, which contribute to our understanding of inelastic and reactive collisions involving hydrogen ions. In an overview of our present theoretical knowledge of various HmD+ collision systems (m + n < or = 5), it is emphasized that although the required potential energy surfaces are well characterized, no detailed treatments of the collision dynamics are available to date, especially at the low energies required for astrochemistry. Instead of treating state-to-state dynamics with state of the art methods, predictions are still based on: (i) simple thermodynamical arguments, (ii) crude reaction models such as H atom exchange or proton jump, or (iii) statistical considerations used for describing processes proceeding via long-lived or strongly interacting collision complexes. A central problem is to properly account for the consequences of the fact that H and D are fermions and bosons, respectively. In the experimental and results sections, it is emphasized that although a variety of innovative techniques are available and have been used for measuring rate coefficients, cross-sections or state-to-state transition probabilities, the definitive experiments are still pending. In the centre of this contribution are our activities on various m + n = 5 systems. We report a few selected additional results for collisions of hydrogen ions with p-H2, o-H2, HD, D2 or well-defined mixtures of these neutrals. Most of the recent experiments are based on temperature variable multipole ion traps and their combination with pulsed gas inlets, molecular beams, laser probing or electron beams. Based on the state-specific model calculations, it is concluded that for completely understanding the gas phase formation and destruction of HmDn+ in a trap, an in situ characterization of all the experimental parameters is required with unprecedented accuracy. Finally, the need to understand the hydrogen chemistry relevant for dense pre-stellar cores is discussed.