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 .

- Mode-hop free operational range from 2147 to 2195 cm
^{-1} - Coarse tuning done by stepping motor
- Fine tuning by application of 5 - 95 Volts to piezo (range 2.2 cm
^{-1}) - Output power 80-100 mW. Linewidth < 30 MHz
- Current modulation to give frequency modulation 10 kHz to 1 MHz (optimum 20 kHz).

Broadband Output of Daylight Solutions 4.6 µm QCL at TAMU

CO, N_{2}O and CS_{2}

Recorded Spectra after development of scanning algorithms

QCL supersonic slit jet spectrum of OC-H35,37Cl with intermolecular hot bands

- Previously developed 2-D and 3-D morphed potential approaches have been effectively extended to 4-D intermolecular treatments of hydrogen bonded systems.
- High resolution infrared and submillimeter analyses of (HI)
_{2}are shown consistent with an unexpected ground state paired hydrogen bonded structure and molecular dynamics quite different for the other homomolecular (HX)_{2}dimers, X= F, Cl, Br. - An extensive microwave and infrared analysis of OC:HI establishes the existence of four isomeric structures within 25 cm
^{-1}of the hydrogen bonded ground state OC-HI. The global minimum, however, is demonstrated to correspond to the van der Waals structure OC-IH. An anomalous structural deuterium isotope effect, ground state isotopic isomerization is revealed based on these investigations.

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 C_{2h} symmetry. The equilibrium dissociation energy is found to be 359 cm–1 with the geometry in Jacobi coordinates of *R _{e}* = 4.35 Å, θ

Vibrational probability distributions for the lowest three states for the four dimer systems values (θ_{1}-θ_{2})/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}

- The combination of high resolution spectroscopy and morphing methodologies provide a powerful tool for not only characterizing weakly bound interactions between molecules but accurately predicting properties not previously suspected. This is in no small part a consequence of the effective treatment of large amplitude vibrations and anharmonicity in this approach.
- In the case of (HI)2, it is demonstrated to have a single global minimum and that the ground state structure has a unique four atom paired hydrogen bond configuration. It is, furthermore, concluded that the HI dimer is also an unusual hydrogen bond in that it is extremely non-linear with an IHI angle of 87.170. Also, morphing predicts that on monodeuteration there is an anomalous structural effect in which the symmetry of the ground state common isotopomer is broken.
- In the case of OC-HI, we have demonstrated that four isomers exist within 0.07 kcal mole-1 and as in Ar-HBr, the ground state structure is demonstrated to be the OC-HI isomer which is differentiated from is global minimum OC-IH structure. Morphing also predicts a different anomalous structural isotope effect in this case, in which the ground state of the complex undergoes isotopic isomerization on deuterium isotopic substitution leading to large structural changes.

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

- 6-D grid of 149,940 points
- aug-cc-pVNZ Basis set
- Augmented correlation consistent polarized valence N zeta
- T – triple
- Q – quadruple

- Coupled cluster singles and doubles with perturbative triples [CCSD(T)]
- Moller-Plesset second order (MP2)
- Hartree-Fock (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.

- Adiabatic Ground Vibrational State PES
- OC-HF
- D
_{e}= 1311(10) cm^{-1} - R
_{e}= 3.598(1) Å

- D
- CO-HF
- D
_{e}= 637(10) cm_{-1} - R
_{e}= 3.444(1) Å

- D

2-D cuts of 6-D PES of OC:HF

Prediction of probability densities for one of the perturbing states in ν_{1} 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:

- Refinements to even further improve the accuracy of predictive capabilities in OC-HF.
- A benchmark potential for critically evaluating less accurate ab-initio and other first principles methods.
- Testing of numerous approximate general models developed to characterize the the properties of hydrogen bonded interactions.
- Basis for generalizing and extending morphing methodologies to larger prototypical systems less tractable for study at the comparable precision possible in the case of OC-HF.