Recent developments using MOCVD methodologies for InP/InGaAs/InAlAs materials have lead to the development of infrared mode-hop free tunable quantum cascade lasers that are now being developed to cover the range 4 – 12 microns while operated under room temperature conditions. These lasers in contrast to semiconductor lead salt lasers which are dependent on electron-hole recombination are based on unipolar multi-subband transitions and have advantages and disadvantages over conventional lead salt diode lasers for spectroscopic purposes. The former lasers should have considerable potential for spectroscopic purposes due to this broadband single frequency mode-hop free operation frequently over 60 cm-1 around their center frequencies and with relatively narrow band free-running linewidths of 30 MHz or less. The capability of operating with these relatively narrow continuous wave single frequency characteristics as well as relatively high average powers of 50 mWatts or more give opportunities for applying a wide range of innovative techniques in spectroscopic studies. These include traditional high resolution spectroscopy in supersonic jet expansions and more esoteric studies involving double resonance and EIT, electromagnetically induced transparency or slow light experiments .
Broadband Output of Daylight Solutions 4.6 µm QCL at TAMU
CO, N2O and CS2
Recorded Spectra after development of scanning algorithms
QCL supersonic slit jet spectrum of OC-H35,37Cl with intermolecular hot bands
Structure of (HI)2 from Four Dimensional Morphed Potential
The HI homodimer was found to have a structural and vibrational properties unlike any other previously studied (HX)2 system, with X= F, Cl and Br. A four dimensional ab initio intermolecular potential has been generated and then morphed using available near infrared and submillimeter spectroscopic data recorded in supersonic jet expansions. The morphed potential is found to have a single global minimum with a symmetric structure having C2h symmetry. The equilibrium dissociation energy is found to be 359 cm–1 with the geometry in Jacobi coordinates of Re = 4.35 Å, θ1 = 43 °, θ2 = 137 °, and Φ = 180°. In (HI)2, the ground state is thus found to be an unexpected four atom paired hydrogen bond.
Vibrational probability distributions for the lowest three states for the four dimer systems values (θ1-θ2)/2 = 470 and φ=180°. All contours are labeled in units of cm–1, with values relative to infinite separation of the monomers.
Morphed (HI)2 PES
Minimum occurs in the symmetric geometry giving rise to unique paired hydrogen bonded structure
Ground State Structure (HI)2 [a very non-linear hydrogen bond]
Possible "Ground State" Structures of OC-HI Isomers within 25 cm-1 (0.07 kcal mol-1)
Investigation of Ground State Molecular Isomerization in OC-HI
Ground State Isomerization of OC-HI to OC-ID on Deuteration is possible due to near degeneracy of OC-HI and OC-IH.
Changes in zero point energy effects on deuteration sufficient to result in ground state isomerization.
Results of preliminary investigations will be reported using Spectroscopic methods morphing of intermolecular potential.
Two-dimensional slices of the morphed interaction potential of OC:HI
All contours are in cm-1
Developing a generalized methodology which generates a vibrationally-complete potential for a hydrogen bond and related interactions capable of predicting such weakly bond intermolecular properties to near spectroscopic accuracy is a challenging problem.
Such potentials could enable all observable intermolecular properties to be accurately predicted from microscopic principles as well as giving the capability of revealing fundamentally new intermolecular phenomena. We are developing such methodologies so that they can be tested in detail and optimized on relatively simple prototypical interactions between molecules prior to being generalized to larger and less tractable systems.
Two examples are given: Ar:HBr and OC-HF.
Morphing the 3-D potential of Ar:HBr to reveal fundamentally new properties of interactions between molecules.
Ar -HBr" Ab-Initio to Vibrationally Complete Morphed Potential.
Conclusion: the ground state and global minimum structures of Ar-HBr are bound differently. The former is hydrogen bound, the latter van der Waals.
Ar-HBr Bending Potential
Conclusion: Ground state bound different from equilibrium (global minimum) structure, a zero point energy effect
We have generated a 6-dimensional vibrationally-complete semi-empirical electronic ground state potential for the hydrogen-bonded interaction OC-HF through application of a morphing methodology. In this technique, an originally generated 6-D ab-initio potential is transformed to a morphed potential through an optimization that accurately fits available experimental data.
Development of a compound-model morphing approach with radial shifting (CMM-RS) is facilitated by an adiabatic separation of high frequency intramolecular vibrations of the complex from the low frequency intermolecular vibrations.
Four parameterized morphing coefficients only are needed to correct for basis set superposition and electron correlation errors while scaling of the potential and shifting of its minimum compensate for inadequacies in the originally generated ab initio potential.
Jacobi coordinates used to describe completely the rovibrational dynamics of OC:HF
Fitting the angular part of ab-initio PES
Radial Interpolation of ab-initio potential
Simplification of 6-D Potential to a 4-D Adiabatic PES through separation of high frequency intramolecular and low frequency intermolecular vibrations
Vibrational Self Consistent Field calculations to generate 4-D adiabatic PES
Rovibrational Hamiltonian for the 4-D adiabatic PES
The morphed adiabatic potential with both monomers in the ground vibrational state is characterized with an equilibrium rotational constant Be = 3345.68(30) MHz, equilibrium center of mass CO to center of mass HF distance, Re = 3.598(1) Å, and equilibrium dissociation energy De = 1310(10) cm-1 giving a ground state zero point energy for the complex of 568(5) cm-1.
The generated CMM-RS potential can also be used to accurately predict numerous properties for OC-HF as demonstrated by: i) The precise experimentally determined ground state dissociation energy D0 = 732(2) cm-1 and the 3rd harmonic of the HF stretching vibration of the complex measured at 10894.46(1) cm-1, both predicted within two standard deviations of error.
Generation of additional characteristics of this complex including states not previously investigated such as vibrational dependence of dissociation energies as well as structural and other properties. Quantitative testing of models developed for investigation of non-covalent interactions is illustrated. Specifically, a detailed evaluation of the widely used empirical Lippincott-Schroeder potential applied to DNA base pairs and other biologically important phenomena is possible through application to the CMM-RS potential of OC-HF.
2-D cuts of 6-D PES of OC:HF
Prediction of probability densities for one of the perturbing states in ν1 OC-DF
Specific application of OC:HF
CMM-RS potential for evaluation of model potentials used in structural biology and nanotechnology
Widely applied Lippincott-Schroeder potential compared with generated CMM PES
Lippincott-Schroeder potential compared with generated CMM PES
The CMM-RS potential has an accuracy of predictability at least 2 orders of magnitude better than any previous OC-HF potential. It is now available for a wide range of applications: