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

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

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)

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

“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) » 10-14 - 10-15 cm2 Subtraction to the shot noise limit: Þ Nmin »107 #/cm3/q.s. … … with 103x improvements accessible w/ cavity/IR comb methods

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

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

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

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

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)

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)

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

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

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