Spectroscopic studies of ionic, Rydberg and ion-pair states of small molecules

• Main Author: Wang, Shiliang
• Published: University of Edinburgh 1999
• Subjects: 543
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.663455

Firstly, a detailed study of the higher Rydberg states of C<SUB>6</SUB>H<SUB>6</SUB> and C<SUB>6</SUB>D<SUB>6</SUB> was performed using mass-resolved polarisation-dependent (2+1) REMPI spectroscopy. Three long series of gerade Rydberg states (two <I>nd</I> and possibly one <I>ng</I>) converging on the first ionisation energy, previously observed up to <I>n</I>=8, were extended up to <I>n</I>=30. By scanning the two-photon energy up to the ionisation limit, coherent two-photon ZEKE-PFI spectra of the ionic states of benzene-<I>h</I><SUB>6</SUB> and -<I>d</I><SUB>6</SUB> were obtained for the first time. The vibrational structure in the ZEKE spectrum is essentially the same as in the (2+1) REMPI spectra of Rydberg states but different from the (1+1') ZEKE spectrum reported previously. Substitution effects and the influence of lowering the symmetry on the Rydberg states were also investigated by comparing studies of C<SUB>6</SUB>H<SUB>5</SUB>F, C<SUB>6</SUB>H<SUB>5</SUB>Cl, <I>p</I>-C<SUB>6</SUB>H<SUB>4</SUB>F<SUB>2</SUB> and <I>o</I>-C<SUB>6</SUB>H<SUB>4</SUB>F<SUB>2 </SUB>molecules. Second, the Rydberg and ionic states of CF<SUB>3</SUB>I have been studied using both REMPI and zero kinetic energy pulse-field ionisation (ZEKE-PFI) photoelectron spectroscopy. The ground state of the ion was characterized using coherent two-photon (one-colour) ZEKE-PFI spectroscopy. The 6p Rydberg states were studied using two-photon REMPI spectroscopy with linear and circular polarised light. The strongest members with Ω=2 were identified and used as resonant intermediates in two-colour (2+1') ZEKE-PFI experiments, which allowed the unambiguous assignment of the majority of the vibrational structure of the intermediate states. Third, multiphoton pathways to the lowest cluster of ion-pair states of ICl and I<SUB>2</SUB>, at energies close to the dissociation limit, are presented. These very high vibrational levels are detected in the anion (Cl<SUP>-</SUP> and I<SUP>-</SUP>) or cation (I<SUP>+</SUP>) channel by pulsed field ionisation. Using a variable time delay before field ionisation, ion pair states up to 50 cm<SUP>-1</SUP> below the dissociation limit are observed to survive for at least 2 μs, indicating a stabilisation process analogous to that operating in high Rydberg electronic states. The analogy between these stabilised ion-pair states and ZEKE (zero electron kinetic energy) states suggests calling them ZIKE (zero ion kinetic energy) states. The atomic ion yield signal is highly structured both above and below the free ion threshold, indicating the role of doorway states which are coupled to the dense ion-pair vibrational manifold near dissociation. This coupling appears to be very efficient and competes successfully with radiative decay and further up-pumping that would result in ionisation. One difference with ZEKE spectroscopy is that these initially prepared states have to undergo both an electronic transition as well as a large angular momentum change, in order to be stabilised. Fourth, molecular photofragment spectroscopy has been used to obtain new insight into the ultraviolet photodissociation of ozone. The formation of O<SUB>2</SUB>(b<SUP>1</SUP>Σ<SUP>+</SUP><SUB>g</SUB>) following absorption in the Huggins band (335-352 nm) of O<SUB>3</SUB> has been observed for the first time. The nascent O<SUB>2</SUB>(b<SUP>1</SUP>Σ<SUP>+</SUP><SUB>g</SUB>) photofragment was detected using two-colour resonance enhanced multiphoton ionisation (REMPI), and the 3sσ<SUB>g</SUB><SUP>1</SUP>Π<SUB>g</SUB> Rydberg state as the resonant intermediate state. Finally, a new VUV generation technique (laser induced plasma), which may be used for spectroscopic studies, has been characterised. Some preliminary results using this new VUV generation source for single photon ionisation and ion-pair production studies of polyatomic molecules (CH<SUB>3</SUB>Br and CH<SUB>3</SUB>I) have been obtained. In addition, high resolution ion-pair formation from CH<SUB>3</SUB>Br using coherent VUV laser radiation generated by four wave mixing has been recorded in both CH<SUB>3</SUB><SUP>+</SUP> and Br<SUP>-</SUP> channels.