COHERENCE-DETECTED FOURIER TRANSFORM MICROWAVE INFRARED SPECTROSCOPY by qxc16070

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									  TUNNELING-SYMMETRY-RESOLVED
  VIBRATIONAL SPECTROSCOPY AND
  DYNAMICS OF THE C2H4-H2S COMPLEX
  MEASURED USING COHERENCE-DETECTED
  FOURIER TRANSFORM MICROWAVE(FTMW)--
  INFRARED SPECTROSCOPY.

MATT T. MUCKLE, JUSTIN L. NEILL AND BROOKS H. PATE
University of Virginia

AND
MAUSAMI GOSWAMI AND E. ARUNAN,
Department of Inorganic and Physical Chemistry,
Indian Institute of Science
 What can FTMW-IR do?

 Outline
 Method                              Tunneling splittings can be
    Coherence Detected FTMW-IR        assigned with pure
      Pulse sequences
      Instrumentation
                                       microwave
 Applications to weekly bound        Can IR be used as a tool for
  clusters                             investigating tunneling and
    Advantages over other methods     looking at dynamics at the
    H2S-C2H4
                                       same time
 Analysis of IR Data
    Band shifts
    Predissociation
Infrared-Microwave Spectroscopy
                 Balle-Flaygare 8-18GHz
                    MW
                   Nd-YAG pumped
                    OPO/OPA tunable IR laser
                   IR-Multipass mirror cavity
                    to increase effective path
                    length.
                   Laser freq. scanned across
                    various MW transitions
                   Pulse valve sample
                    introduction
                     Supersonic expansion
                      cooling to ~2K
Experimental Setup
 Coherence Detected FTMW-IR




 MW-IR-MW Pulse sequence
 1st MW “π/2” polarizes sample and eliminates population
  difference
 IR Pulse transfers population out of monitored state
 2nd MW “-π/2” pulse cancels remaining coherent light
 Population is converted into a coherence
   Induced population difference causes the second pulse to be greater or
     less than an exact -π/2 pulse
No Laser Resonance
Laser Resonant with Lower State
Laser Resonant with Upper State
Applications Of IR To Weak Clusters


Previous Hydride Stretch Work    Current
 Bolometer Detection             Cavity FTMW (pulsed jet)
  (Miller, Fraser, Scoles)          High sensitivity of weak
                                     clusters
 Direct Absorption (Nesbitt)
                                    Lower Resolution
   High Resolution
                                       .02cm-1 (600Mhz)
     .001-.0001cm-1 (3-30MHz)
                                         Limits Lifetime Determination
   Limited to small clusters
                                    IR-MW double resonance
                                      Simplifies spectrum (only two
                                        base J states)
Applications of FTMW-IR to C2H4–H2S



 Band Origin Shifts
   Lifetime
   Complexation effects
 Mode-Specific
  Predissociation dynamics
 Tunneling dynamics
 Excited Geometry
 Competition of IVR and
  Predissociation
H2S-C2H4 Dimer



                                             Loosely Bound
                                              (0.3196kcal/mol)
                                             4 Tunneling Components
                                                 A       26GHz
                                                 B       1972.90MHz
                                                 C       1866.69MHz
                                                 Dj      14.3kHz
                                                 Djk     1.061MHz

                                      º6-311++G** with ZPE correction
       M. Goswami, PK Mandal,DJ Ramdass, E Arunan Chem Phys Lett 2004
                               Tunneling Components


Larger splitting(10MHz) from H2S
Proton Exchange




Smaller splitting(1MHz) C2H2
Rotation around C=C axis
S-H Stretching Modes

 2 IR active stretching modes in free H2S
   Band Origins
     2614.4080cm-1 symmetric
     2628.4551cm-1 asymmetric
 Expectations
   Complexation should make a strong red shifted
    “bound” and a weakly shifted “free” S-H stretch
   (shift)2 should be proportional to lifetime*


                             *R.E. Miller, Science, 240 (1988) p.447.
                             *Le Roy, J Phys Chem, 95 (1991) p. 2167.
S-H Stretches
S-H Stretch   K1
  Band Origins
                   Monomer          Complex    Shift   Type
  LL (1L)          2614.335         2607.88    6.455   A
  LU (1U)          2628.431         2622.18    6.251   A
  UL (2L)          2614.335         2608.30    5.035   A
  UU (2U)          2628.431         2625.72    2.711   A



Symmetric stretch predicted to be redshifted
farther than the asymmetric

Both bands have a nearly identical shift
Predissociation

             Broader Linewidth in
              Lower band
               ~ 0.05cm-1 linewidth in
                  both K0 and K1
                   100ps lifetime
               Does NOT agree with
                  Miller’s rule
                   (shift)2 lifetime
Experimental vs Predicted Lifetime

             R.E. Miller, Science, 240 (1998) p.447.



                                                       100ps lifetime would be a
                                                       300cm-1 shift in band origins

                                                       6.53 and 6.28cm-1 shifts from
                                                       Monomer

                                                       Large deviation from Miller’s
                                                       Rule

                                                       Deviation in upperstate
                                                       rotational constant may speed
                                                       up reaction rate (Le Roy++ )



                                             ++ Le Roy, J Phys Chem, 95 (1991) p. 2167
Combination Band

  ~50cm-1 shift from S-H
   stretch
  Possible coupling to
   intermolecular mode
  Same lifetime
   broadening in lower
   band
C-H Stretches

Expectations
 No large shift to Band
  Origins
    H2S not directly interacting
     with C-H mode
 Tunneling
    No shift for large splitting
    Different shifts for each
     mode on small splitting
C-H Stretch
 Band Origins

      C2H4 monomer Band Origins
         2988.643cm-1 **
         3104.887cm-1 **
         Very little deviation from complex
           Indicative of small binding interaction
      Identical B type Origins ( Lower Band)
      Thee C type bands (Upper band Lower doublet)
LL             LH             HL             HH                   Type
     2987.12        2987.12        2987.12        2987.12       B
     3103.31        3103.35        3103.42        3103.51       C
     3103.56        3103.56           -              -          C
     3104.49        3104.40           -              -          C
                                                            **ref: J. Mol. Spectrosc., 185, 31-47 (1997)
Lower Band C-H stretch Coupling




                Perturbations
                   Not coupled to S-H modes
                     No perturbations on S-H bands
                   Not coupled to C2H2
                     No perturbations in monomer
                   Most likely coupled to
                    intermolecular mode
                      No lifetime broadening!

								
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