Terahertz laser spectroscopy of molecular ions

The use of the Berkeley Terahertz Spectrometer has been extended from the study of neutral water clusters in a pulsed slit molecular beam expansion to the study of ions between 50 and 105 cm-1. Two ion sources have been employed in an attempt to observe the ro-vibrational spectra of H5O2+. The first was the pulsed slit discharge molecular beam source, which was used near 50 cm-1 , and the second was an AC glow discharge, used with the velocity modulation technique at frequencies below ∼100 cm-1. H3O+ and ArH+ were used as test ions for the purpose of optimizing and characterizing these two ion sources.; The pulsed slit discharge molecular beam source yielded a H3O + signal with a S/N of ∼200/1 for the Q(3,3) transition of the pure inversion motion. This source was used to scan with a Ne/D2O mixture between 48 and 57 cm-1 and produced ∼450 transitions with S/N ratios as large as 3000/1. Of these transitions, 81 were assigned to electronic ground state hotband transitions of D2O. In the remaining ∼370 transitions no trends were discernible and it was impossible to determine whether an individual absorption arose from an ionic or a neutral species. Therefore, these transitions were not assigned.; Velocity modulation spectroscopy is a well-established method of effective ion selective detection, but in the terahertz region it has seen extremely limited use. This is due to complications arising from the competition between Doppler broadening and pressure broadening that occurs below 100 cm -1, which decreases the modulation depth achievable by VM in this regime. Nevertheless, VM has been successfully applied in this region, yielding a S/N ratio of 770/1 for the J = 5 ← 4 rotational transition of ArH+ at ∼100 cm-1. In the process of characterizing the performance of VM spectroscopy in this region, pressure broadening studies of ArH+ were carried out on both the J = 5 ← 4 and J = 3 ← 2 transitions of ArH+ and compared to the predicted Langevin result. For reasons related to complexities involving the velocity modulated Voigt lineshape, the experimental and theoretical pressure-broadening coefficients differed by an order of magnitude. It is concluded that this disparity is artificial and does not indicate a breakdown of Langevin theory.; The VM technique was also used to observe the Jka = 7 5+ ← 65- transition of the pure inversion of H3O+ at ∼100 cm -1. After optimizing the glow discharge plasma conditions on this transition, the frequency regions from 80--83 cm-1 and 101.6--104.4 cm-1 were scanned in search of other ion transitions that could be attributed to the H5O 2+ ion.; Finally, and separately from the studies of ions, the observation and assignment of the lower acceptor switching fork of the ka = 0 ← 1 transition of (H2O)2 was made. Other frequency regions were also scanned in search of more water dimer transitions, and a compilation of all high resolution terahertz spectroscopy performed on water clusters is tabulated.