## Research Accomplishments of Kevin Lehmann1. Provided the first correct description of the nature of the reaction
between H 2. Performed one of the first Resonance Enhanced, Multiphoton Ionization
Studies of a diatomic molecule, leading the discovery of several new
electronic states of I 3. Ph.D. thesis work was the first systematic attempt to use high resolution overtone spectroscopy to study the nature of intramolecular vibrational energy redistribution (IVR). This work demonstrated that rapid IVR is not universal in small polyatomic molecules, even at an excitation of ~50% of a C-H bond dissociation energy. This work also set a standard for the careful comparison of polyatomic vibrational spectra at high energy with theoretical predictions based upon variational calculations of the ro-vibrational dynamics. (4-8,10,14) These studies where later extended by Lehmann to high energy using novel spectroscopic methods of increasing sensitivity, and has most recently allowed the study of vibrational states of HCN with ~75% of the dissociation energy (137). Even at this energy, which is twice the barrier for the HCN <-> HNC chemical isomerization, the molecule does not isomerizes, despite the fact that statistical reaction rate theory predicts sub-ps isomerization. 4. Demonstrated the formal equivalence of Harmonically Coupled, Anharmonic
Oscillator Model of Mark Child and others with the older Darling-Dennison
resonance model. This work demonstrated how local mode vibrational dynamics
arises from a normal mode treatment of the anharmonic oscillator problem.
(9). The work was then extended to show how a Darling-Dennison type resonance
model can be constructed for any molecule (18, 33). Later work demonstrated
how the local ro-vibrational limit naturally arises when the local mode
tunneling rate becomes longer than the rotational precession rate and
how this leads to a further reduction in tunneling rates and the transformation
of the spectrum into four fold degenerate clusters of levels (43, 54).
These spectral structures were later discovered in overtone spectra of
H 5. Developed the highly sensitive and state specific method of Microwave
detected, Microwave-Optical double resonance (MD-MODR) (13). This method
was used for an extensive study of the entire overtone spectrum of NH 6. The MD-MODR method was used for the first systematic study of the
statistical character of the spectrum of NO 7. The determination of the bending potential of HCP over a then unprecedented
range of bending angle by use of dispersed emission spectroscopy (15).
This work set the stage for later Stimulated Emission Pumping experiments
performed by Robert Field and collaborators (38). This SEP work was greatly
added by a comprehensive study of the electronic spectrum of HCP in a
cold jet (T 8. A series of studies were made to provide, for the first time, accurate
systematic overtone band intensities of a polyatomic molecule (21, 24,
30, 32, 34). Theoretical modeling showed that the data could not practically
be used to determine polyatomic dipole moment functions, but did provide
an exceptionally sensitive test of the quality of 9. In collaboration with G. Scoles and others, Lehmann completed a series of sub-Doppler, Molecular Beam Spectroscopy studies of IVR in medium to large molecules (37, 39,44-47, 49, 59, 60, 68-72, 75, 77, 78, 80, 83, 90, 91, 97, 98, 116. 130). This work built upon previous and concurrent studies by the groups of Perry and Nesbitt, but extended beyond it in terms of spectral resolution, sensitivity, and range of vibrational energy studied. This work developed MW-IR and IR-IR double resonance spectroscopic methods to assign spectra that were too highly fractionated for assignment, even at ~ 5 MHz linewidth found in the molecular beam spectra, and provided the first sub-Doppler spectra in the second C-H overtone region (68, 71). Resonance build-up cavities were introduced to greatly increase the one beam optical intensity and improving the lineshape. This work provided the first demonstration of rigorously homogeneous Lorenztian lineshapes due to IVR, as expected in the large molecule statistical limit (46). The work unequivocally demonstrated that density of vibrational states has little to no effect on the rate of IVR, except that a minimum density is needed to provide a quasi continuous `bath' for relation on the natural decay time scale of the chromophore. The work demonstrated that delocalized combination states can relax more slowly than nearly isoenergetic pure overtone states (68), which is the opposite of what had been the accepted wisdom based upon models of small molecules with only strongly interacting modes. Experiments on the first overtone band of benzene have demonstrated that IVR dynamics can be richly hierarchical, stretching over four orders of magnitude in time and still not reaching a truly statistical final point (90, 116). A unique ‘two cavity’ IR-IR double resonance instrument was built (117), providing spectra with ~15 MHz linewidth spectra of molecules with ~2 eV of vibrational energy. This instrument was used to investigate isomer tunneling in acetylene about the barrier for formation of vinylidene, establishing that the rate of this process is at least four orders of magnitude slower than the RRKM prediction of this rate (112). 10. The IVR work was supported by a series of developments of theoretical methods for the analysis of spectra. The Molecular Symmetry group for three equivalent rotors was worked out (41), and a general program for generating Molecular Symmetry groups written. A vastly superior numerical algorithm for the Lawrence-Knight (L-K) deconvolution procedure was presented, which included for the first time error estimates for the resulting molecular parameters (45). The L-K deconvolution allows one to determine the coupling matrix elements between the optically excited state and the background of other ro-vibrational states at the same energy. It was demonstrated that high order anharmonic interactions could be quantitatively predicted from chains of cubic and quartic near resonances. (52). A more computationally efficient method to calculate vibrational density of states in nonseparable systems was developed. (61). 11. Lehmann was the first to exploit the method of Cavity Ring Down Spectroscopy (CRDS) to make quantitative measurements of absorption spectra, including the first treatment for the effects of finite laser resolution (65, 79, 84). This work was also the first to provide a correct analysis of the fundamental noise sources in this method and how it determines ultimate sensitivity limits. The first quantitative studies of spectral lineshapes using CRDS, including line mixing effects, were also demonstrated and used to study collisional transfer of optical coherence (85, 96). The first general theory for the CRDS technique, properly including optical interference and dispersion effects was published (87, 94), demonstrating that limitations of the incoherent `photon bullet' model previously used. The first experiments extending CRDS to continuous wave excitation sources was performed in Lehmann's laboratory (122, 133) and the advantages of these methods first theoretically derived, leading to a US. Patent #5,528,040. The first sub-Doppler CRDS experimental method was developed in collaboration with NIST (110). A novel cavity based upon Brewster Angle roof prisms has been designed and demonstrated, allowing this high sensitivity method to be applied over a broad spectral range in a single cell (103). The first commercial CRDS based analytical instrument was developed based upon the Ph.D. thesis work of John Dudek (Princeton, 2000). A Fiber-optic loop version of CRDS was developed and used for chemical sensing (135), detection in strain (140), and single cells (143). 12. Lehmann joined with Scoles in the ongoing spectroscopic study of
molecules solvated in nanometer scale He droplets. They have provided
a spectroscopic analysis of the quartet state of Na 13. Lehmann has had several notable theoretical accomplishments in the new field of helium nanodroplet spectroscopy. He developed a theory for the motion of solute molecules and ions in liquid He droplets (100, 101, 118) that has provided a framework for the interpretation of He droplet spectra taken at Princeton and other laboratories. He has developed a superfluid hydrodynamic model that quantitatively reproduces the observed large increases in effective moments of inertia of molecules dissolved in He (104, 129). This theory has also been applied to the effective mass of alkali cations in helium and found to be in quantitative agreement with Monte-Carlo calculations (127). A model was developed that for the first time reproduced the enormous effective centrifugal distortion constants of molecules in liquid helium (120). This work demonstrated that this arises not from classical centrifugal effects but due to a twist boundary condition that was earlier discussed, in the context of the “rotating bucket experiment” by A. Leggett. Lehmann provided theoretical estimates for the formation and decay of general, curved vortices in helium nanodroplets (134). This work demonstrated the importance of angular momentum conservation constraints, which had been neglected in previous discussions. He also provided a thorough study of the microcanonical thermodynamic properties (including angular momentum conservation) for helium droplets (132). This work let to the surprising result that in the rotational temperature of a molecule dissolved in a droplet is not the same as that of the droplet even when they are in equilibrium as a combined microcanonical-angular momentum conserving ensemble (136). The derived helium droplet thermodynamic functions allowed Lehmann to study the evaporative cooling of droplets including angular momentum constraints (139). This recent work has demonstrated that the droplets cool to essentially the same temperature as previously predicted (ignoring angular momentum constraints) and observed experimentally, but that the energy and angular momentum of the droplets is one to two orders of magnitude higher than for a canonical ensemble at the same temperature. |

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