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42867-AC6
Detecting the Approach to Delocalization on the HNC <-> HCN X 1Sigma+ Potential Surface Using SEP-Millimeter Wave Stark Spectroscopy
The primary goal of this program is to detect the onset of delocalization on the S0 HNC-HCN potential energy surface by observing and identifying barrier-proximal states. Barrier-proximal states are energetically located near the isomerization barrier and have amplitude localized along the minimum energy isomerization path, but represent only a small subset of the more numerous and less interesting highly excited vibrational states. Therefore, they are difficult to identify with the traditional spectroscopic method of assigning and fitting all vibrational levels. The large amplitude motion embodied in the rare barrier-proximal states, however, causes changes in the electronic properties of the molecule that provide markers to distinguish them from other states. In the HCN<->HNC system, the magnitude of the electric dipole moment, |µ|, is a particularly effective diagnostic because it is significantly reduced (to ~1 D) by the cancellation of the two oppositely signed dipole moments in the HCN (~ +3 D) and HNC (~ - 3 D) configurations. Thus, a Stark effect measurement in a highly vibrationally excited state will immediately reveal the extent of delocalization.
Nuclear quadrupole hyperfine structure is another electronic property capable of identifying barrier-proximal states. Hyperfine structure arises from the interaction of the nuclear quadrupole moment of the nitrogen nucleus with the gradient of the electronic field. In the J=1-0 rotational transition of HCN or HNC, hyperfine splitting gives rise to a triplet of lines. In HNC, however, the hyperfine splitting is much smaller and appears unresolved as a single line under Doppler-broadened conditions. We have recently improved the resolution of our mm-wave jet spectrometer by propagating the mm-wave radiation coaxially with the molecular beam. This geometry substantially reduces Doppler and transit-time broadening, allowing all members of the HNC hyperfine triplet to be resolved. These measurements are the first laboratory measurements of the hyperfine structure in HNC and demonstrate unambiguously that the sign of the hyperfine coupling constant (eQq) is positive, in contrast to the negative value of (eQq) found in HCN. This observation has important implications for astrophysical measurements of HCN and HNC abundance ratios in interstellar clouds. To aid these measurements, we have also measured the hyperfine structure of several HNC isotopomers: D14NC, H15NC, HN13C, and D15NC. More revealing, however, is the change in hyperfine structure as a function of bend-excitation in HCN and HNC. Bend excitation in HCN leads to a nearly monotonic increase in hyperfine splitting, whereas bend excitation in HNC causes the hyperfine structure to collapse and reverse sign. This strong dependence of the hyperfine structure on the extent of bend excitation is consistent with ab initio calculations performed in our group.
Before we can probe electronic properties of barrier proximal states, we must populate these highly excited vibrational levels. Based on calculations of the HNC excited state geometry, we believe that these bending levels are better accessed through stimulated emission pumping (SEP) experiments originating on the HNC side of the potential surface rather than on the HCN side. The first excited electronic state of HNC, however, has not previously been observed, in part because HNC is an unstable molecule and the excited state is predicted to be predissociated. We have successfully produced HNC in a discharge jet and have undertaken a systematic search for the excited state of HNC using laser induced fluorescence (LIF) and photofragment fluorescence excitation (PHOFEX) of the CN fragments. Although the region between 193 and 205 nm is rich with spectral features, none can be attributed to HNC. Indeed, all the LIF features and most of the PHOFEX features can be assigned to HCN hot bands. The spectral interference of HCN and other molecules produced in the discharge limit our sensitivity and hamper our search efforts. Thus, we are exploring alternative methods for HNC production.
As a test of the PHOFEX detection scheme, we have observed the electronic spectrum of isocyanogen (CNCN) for the first time. Our measurements indicate that the first excited electronic state is bent and rapidly predissociative, in agreement with ab initio calculations. Unfortunately, the rate of predissociation is too rapid to exploit this electronic state as an SEP intermediate level.
The research funded by this grant has stimulated new ideas and experiments in this laboratory. In particular, the idea of using electronic properties to identify barrier proximal states has been extended to other isomerization systems, including acetylene<->vinylidiene. Moreover, the experience with millimeter wave technology has motivated new methods and applications for studying other dynamic processes, such as inelastic scattering in molecular beams and electron<->ion-core interactions in Rydberg states. The grant has supported a visiting professor on sabbatical, who has taken the experience gained at MIT to set up a new laser laboratory at the University of Evansville, a primarily undergraduate institution. The grant has also supported the research of a graduate student and a postdoctoral fellow.
