Search

Titanium isotope ratios have been measured by resonance ionization mass spectrometry (RIMS) with special emphasis on the nature of laser-induced isotopic selectivity. A pronounced wavelength dependence of even mass isotope ratios is caused by large nuclear volume effects near the magic neutron number 28 in [.sup.50Ti]. Optical isotope shifts, ranging from 0.07 to 0.21 [cm.sup.-1], between [.sup.50Ti] and [.sup.46Ti] were measured for several transitions. The [.sup.50Ti]/[.sup.46Ti] and [.sup.48Ti]/[.sup.46Ti] ratios, nevertheless, exhibited only mass-dependent fractionation, in which the lighter Ti isotopes were enriched by '2.5%/amu, when the laser operating parameters were properly controlled. Odd--even mass isotopic selectivity in the resonant ionization process was also examined for several transitions as a function of the laser polarization state and intensity. Under saturating conditions for a [delta]J = +1 transition and a high degree of laser depolarization for a [delta]J = 0 transition, the odd--even isotopic enhancement was reduced below the 2% level. The Ti isotope data agree with our previous results for Os and indicate that, by a careful choice of resonance transitions and laser operating parameters, isotope ratios can be measured accurately and reliably with RIMS., The estimation of titanium isotope ratios by resonance ionization mass spectrometry is discussed. Laser induced isotopic selectivity was specifically analyzed by the measurement of optical isotope shifts at different titanium transition values. The dependence of RIMS effectivity and accuracy on the proper selection of resonance transitions and laser operating parameters was demonstrated.

The relative abundances of singly deuterated methanol isotopologues, [CH 2 DOH]/[CH 3 OD], in star-forming regions deviate from the statistically expected ratio of 3. In Orion KL, the nearest high-mass star-forming region to Earth, the singly deuterated methanol ratio is about 1, and the cause for this observation has been explored through theory for nearly three decades. We present high-angular resolution observations of Orion KL using the Atacama Large Millimeter/submillimeter Array to map small-scale changes in CH 3 OD column density across the nebula, which provide a new avenue to examine the deuterium chemistry during star and planet formation. By considering how CH 3 OD column densities vary with temperature, we find evidence of chemical processes that can significantly alter the observed gas-phase column densities. The astronomical data are compared with existing theoretical work and support D-H exchange between CH 3 OH and heavy water (i.e., HDO and D 2 O) at methanol's hydroxyl site in the icy mantles of dust grains. The enhanced CH 3 OD column densities are localized to the Hot Core-SW region, a pattern that may be linked to the coupled evolution of ice mantle chemistry and star formation in giant molecular clouds. This work provides new perspectives on deuterated methanol chemistry in Orion KL and informs considerations that may guide future theoretical, experimental, and observational work.

We demonstrate that halogenated methane (HM) two-dimensional (2D)-terahertz-terahertz-Raman (2D-TTR) spectra are determined by the complicated structure of the instrument response function (IRF) along ω 1 and by the molecular coherences along ω 2 . Experimental improvements have helped increase the resolution and dynamic range of the measurements, including accurate THz pulse shape characterization. Sum-frequency excitations convolved with the IRF are found to quantitatively reproduce the 2D-TTR signal. A new reduced density matrix model that incorporates sum-frequency pathways, with linear and harmonic operators, fully supports this (re)interpretation of the 2D-TTR spectra.

Fundamental properties of molecular liquids are governed by long-range interactions that most prominently manifest at terahertz (THz) frequencies. Here we report the detection of nonlinear THz photon-echo (rephasing) signals in liquid bromoform using THz-THz-Raman spectroscopy. Together, the many observed signatures span frequencies from 0.5 to 8.5 THz and result from couplings between thermally populated ladders of vibrational states. The strongest peaks in the spectrum are found to be multiquantum dipole and 1-quantum polarizability transitions and may arise from nonlinearities in the intramolecular dipole moment surface driven by intermolecular interactions.