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Imaging Detailed Photodissociation Dynamics and Construction of An Apparatus for Ultra Cold Chemistry Study

Author: ZhangCuiMei
Tutor: Chen
School: University of Science and Technology of China
Course: Physical and chemical
Keywords: ion imaging REMPI Dynamics Helium nano droplet
CLC: O643
Type: PhD thesis
Year: 2013
Downloads: 20
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Abstract

This thesis consists of two parts:one is the studies of the gas phase photodissociation dynamics by the velocity map ion imaging technique. The photodissociation dynamics of the formic acid molecule and the CS2+cation were discussed in detail. The competition of direct dissociation, internal conversion, and intersystem crossing was confirmed in the formic acid photodissociation at the UV region. The mode-selective effect for the photodissociation of the GS2+cation was observed. The other is the construction of the novel (pulsed) helium nano droplets spectrometer. The photodissociation dynamics of the OCS molecule doped helium droplets and N2++H2reaction were carried on the state of art apparatus.Part I:Imaging detailed photodissociation dymatics of HCOOH molecule and GS2+cationPhotodissociation dymatics of HCOOHThe photodissociation dynamics of formic acid have been studied using the velocity map ion imaging at the230-244nm region. The OH REMPI spectrum from the photodissociation of formic acid at244nm has been recorded by resonance enhancement multiphoton ionization (REMPI). The spectrum shows low rotational excitation (N≤4). By fixing the probe laser at the specific rotational transitions, the resulting OH images from various dissociation wavelengths have been accumulated with dc slicing ion imaging. The translational energy distributions derived from the OH images imply that about half of the available energies go to the photofragments internal excitation. With combined experimental observations and previous theoretical calculations, the possible mechanism for244nm photodissociation is internal conversion(IC). For230nm photodissociation, there was the competition among the direct dissociation, intersystem crossing (ISC), and internal conversion (IC).Photodissociation dynamics studies of CS2+via the A2Ⅱu(v1,v2,0)stateThe vibrationally mediated photodissociation of CS2+cations via the A2Ⅱu(v1,v2,0)state and the corresponding UV one-photon photodissociation have been studied by means of the velocity map ion imaging technique. The pure CS2+X2Ⅱg (0,0,0) cations were prepared by a (3+1) resonance enhanced multiphoton ionization process. The photo-fragment excitation spectrum of S+was recorded by scanning the photolysis laser via the A2Πu(v1,v2,0) state. The (1+1) photodissociation images of S+photofragments by the mode-selected A2Πu(v1,v2>0) levels, by fixing the photolysis laser wavelength at the specific vibrational state, from numerous vibrationally mediated states have been accumulated. The translational energy release spectra derived from the resulting images imply that the co-fragments, CS radicals, are both vibrationally and rotationally excited. The β parameters in the dominating TER regions with high J rotational distributions are nearly isotropic, while those in the low J rotational distribution regions have parallel characters. In the scheme of UV onephoton excitation that reaches the same energy region as in the (1+1) photo-excitation scheme, we obtained S+images. Comparing the vibrationally mediated photodissociation with one-photon photodissociation observations, clear evidence of vibrational state control of the photodissociation process is observed.Part Ⅱ:Novel (pulsed) helium nano droplets spectrometerConstruction of the novel (pulsed) helium droplets apparatusWe constructed a new pulsed helium nano droplets machine. The droplets were generated by expansion of the pure helium through the cryogenic valve attached to a closed-cycle cryostat. The mean size of helium droplets can be controlled between103to105helium atoms by tuning the backing pressure (10-40bar) and temperature (10-30K) and the number density is1015-1016/cm3. Compared with the continuous-flow beam source, the density of droplet is at least one order of magnitude higher, which offers the opportunity to combine the system with the commercial pulsed laser to study chemical reactions inside of the superfluid helium at ultra-low temperature. The performance for the system has been checked by studying the photodissociation of CH3I doped droplets with the velocity map imaging technique and the depletion spectrum of benzene doped helium droplets.Photodissociation Dynamics of OCS in Helium DropletsThe photodissociation dynamics of carbonyl sulfide in helium droplets has been studied by means of time-sliced velocity map imaging. The CO fragments were detected by (2+1) resonance enhanced multiphoton ionization. It is found that in the helium-droplets environment rotational cooling is much more efficient than vibrational cooling. The velocity map images for both CO+(v=0) and CO+(u=1) exhibit nearly isotropic angular distributions. The kinetic energy distributions show that most of the translational energies are relaxed in the finite-sized superfluid helium system. However, the average translational energies of the CO (v=1) images are higher than those of the CO (v=0) images.The relevant mechanism has been briefly discussed.Ion molecule reaction N2++H2in helium dropletsUsing the electon impact (El) ionization of the doped helium droplets, we studied the ion-moleule reactions inside the helium droplets. The N2++H2reaction is the benchmark system which is more related to the upper atmosphere and interstellar medium. By co-doping the nitrogen and hydrogen molecules inside the droplets, we studied the reaction products followed by the El ionization of the doped droplets. The preliminary experimental results indicate that the helium atom firstly ionized with the election impact. Then the nitrogen molecules are ionized with the charge transfer from the helium ions. In order to obtain the insight of the reaction dynamics, we studied the dependence of the reaction products with the concentration of the reactants. The results imply that the reaction channel is more favorable with the exothermic.

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