1. Spin-Rotation Spectroscopy and Dynamics of Hydroxymethyl Radical (H2COH) Chih-Hsuan Chang, Fang Wang, and David J. Nesbitt
JILA
Illinois Symposium on Molecular Spectroscopy
University of Illinois
Champaign, Illinois
June 19, 2014
Work done at
JILA/Department of Chemistry and Biochemistry
National Institute for Standards and Technology
University of Colorado
Boulder, CO

2. In Search of Elusive Chemical Reaction Intermediates
Universal need for universally sensitive radical detection methods
High resolution laser spectroscopy: Powerful tool for remote sensing, kinetics, diagnostics, etc. of radicals under complex, real world kinetic conditions

3. Sub-Doppler Slit-Jet Spectrometer
Tunable high resolution IR (1-4 mm with DFG, 4-10 mm with QCL)
Quantum shot noise limited sensitivity (< 1 x 10-5 single pass)
Sub-Doppler molecular linewidths (» .002 cm-1)

4. Slit Jet Supersonic Discharges
localized
discharge
Rational “synthesis” of radicals
High densities at slit orifice (» 1013 radicals/cm3)
Rapid cooling (» 1-2 ms) into lowest energy quantum states
Maximally simplified spectroscopy of highly reactive “hot” radical intermediates at “cold” jet conditions (15 K)
Cl2 + e  2Cl + e
Cl + CH3OH  HCl+ CH3O
Cl + CH3OH  HCl+ CH2OH
CH2OH + He/Ne  CH2OH + He/Ne

5. “Simple” Beer’s Law Physics (at the quantum shot noise limit)
100 mm
5 cm
“Absorbance = density (r) x cross section (s) x path length”
Slit Þ 100x enhancement in path length
Slower 1/r vs 1/r2 density drop off in slit: Þ 100x enhancement in r
Sub-Doppler resolution: Þ 10-20x increase in s(IR) » cm2
Subtraction to the shot noise limit: Þ Nmin »107 #/cm3/q.s. …
… with 103x improvements accessible w/ cavity/IR comb methods

6. Previous Infrared CH2OH Studies
CH2 sym a
IR/REMPI studies by Reisler et al.
All 3 OH/CH stretch bands observed at 0.5 cm-1 resolution…
…sufficient to see partially resolved rovibrational structure…
CH2 asym b
OH str b a 6

7. i) Tunneling Dynamics 2–fold barrier to internal rotation
Predicts doubling of spectral bands due to transitions from lower (0+) and upper (0-) tunneling levels…
…split by sums (or differences) of ground/excited tunneling splittings

8. ii) Nuclear Spin Statistics
C2v(M) MS Group
(b)x
(c)y
(a)z
C2v(M) E
(12)
(12)* E*
C 2v
C 2
v(xz)
v(yz) A1 1 A2 1 B1 B2
Uncoolable Ka = 0, 1 nuclear spin states always present in the slit discharge expansion
(CH2 asymm) = B 1
(OH str) = A1
(CH2 sym) = A 1
Tunneling
Rotation
3:1

9. iii) Sub-Doppler Access to Electron/Nuclear Spin Dynamics (e.g., CH2D)
J = N + S
F1 = J + IH2
F2 = F1 + ID
Unsplit rotational level
Spin-rotation splitting
Nuclear hyperfine splitting

10. Fundamental OH/CH Stretch Bands
RJ10
CH2 sym (A1)
CH2 asym (B1)
OH str (A1)
a(A1)
b(B1)
b(B1)
a(A1)
TE01
TI03
Ka=0(3)
Ka=1(1)
Ka=0(1)
Ka=1(3)
0+(A1)
0(B1)
1+(A1)
1(B1)
a/b-type
Ka=0(3)
Ka=1(1)
Ka=0(1)
Ka=1(3)
0+(A1)
0(B1)
2(B1)
2+(A1)
b-type
3+(A1)
3(B1)
Ka=1(1)
Ka=1(3)
Ka=0(3)
Ka=0(1) 
a-type
Ka=1(1)
Ka=1(3)
Ka=0(3)
Ka=0(1)
0+(A1)
0(B1)

11. Sample pQ1 Branch Data (CH2 Asym Stretch)
202  211 (3)
303  312 (3)
101  111 (3)
404  413 (3)
Clear pairs of well resolved +/- tunneling transitions…
…as well as additional spin-rotation structure
101  111 (1)
202  211
303  312 (1)
404  413+ (1)

12. Spin-Rotation Fine Structure
Sub-Doppler collimation in the slit jet…
…yields resolved spin-rotation structure (and often hyperfine broadening)
Novel spectroscopic access to fine/hyperfine information in the near IR

13. Summary/Conclusions
New methods for sub-Doppler, jet-cooled, infrared high res spectroscopy of highly reactive radicals
H2COH spectra observed in all three high frequency vibrational modes (CH2 sym, CH2 asym, OH)
Nuclear spin statistics, “uncoolable” Ka = 0,1 states
Spin-rotation fine structure interactions fully resolved and analyzed with a Watson/Spin-Rotation Hamiltonian
Potential for “bootstrapping” chirped mwave, mm, and sub-mm spectral searches in the ISM

14. Acknowledgements Chih-Hsuan Chang Fang Wang Melanie Roberts (SUNY)
Chris Carpenter
NSF
DOE
NIST