COLLISIONALLY-MEDIATED SINGLET-TRIPLET CROSSING IN ã1A1 CH2 REVISITED: (010) COUPLING ANH T. LE, GREGORY HALL, TREVOR SEARSa Department of Chemistry Brookhaven National Laboratory Upton, NY, USA International Symposium on Molecular Spectroscopy 69th Meeting, June 16-20, 2014 Champaign-Urbana, Illinois a. Department of Chemistry, Stony Brook University, Stony Brook, New York 11794

Previous work CH2 bending potential a state X state |S> |T> CIISC occurs only through the few pairs of levels which are mixed, and thus have partial singlet and partial triplet character. These pairs then act as “gateways” for the relaxation between spin-rotation-vibration levels associated with different electronic states.

Types of experiment to study mixed state pairs : Doppler limited spectroscopy to identify the perturbed lines from level shifts Saturation dip spectroscopy to study the ortho hyperfine structure: No hyperfine splitting unless both both I and S are non-zero Kinetics At 1MHz resolution the hyperfiine component the ortho level in the ã1A1 state remain the same while the ortho level in the X3B1 state split due to the coupling between the unpaired electron spin (S) with 1H nuclear spin (I). Bley and Temps, J. Chem. Phys., 98, 1058 (1993) kISC = S 2fi krot b2 (1-b2)

Perturbed lines from level shifts? Kobayashi et. al [J. Chem. Phys. (2006)] rotational analysis of ã(010). b1B1(030)0-ã1A1 (010)1 ˜ 717 515T 515S 111 212 313 414 515 616 515 the only one that has triplet partner identified Only 5 of 54 lowest levels >.1 cm-1 deviation from rigid rotor fit

How about kinetics of ã1A1(010)? Single exponential decay on ã1A1(010) after the initial thermalization. Slope=0.115 ms-1Torr-1 Slope= 0.155 ms-1Torr-1 Total rate afte Plotting the observed decay rate vs helium pressure measure the intersystem crossing rate constant from the slope get nearly identical slope between v=1& v=0. In each case the para rate is 30% faster than the ortho. ã1A1(000) kinetic ã1A1(010) kinetic Kisc rate is similar for ã1A1(000) & ã1A1(010) Hard to reconcile with the small number of identified perturbed levels in ã1A1(010)

Experiment setup CH2CO+308nm CH2+CO EOM Φ FM Receiver 1.5m

Time wRF=192MHz l Unmixed state no hyperfine splitting EOM

ã1A1(010) Sub-Doppler measurement on 616 line 1 broadened hyperfine feature or 3 unresolved features? 313 Linewidth(FWHM): 2.4 MHz Out of those lines only 616 515 717 are in the region that are in the Msquare laser. 616 is the only possibility to have any splitting Linewidth 2.4MHz Linewidth 6MHz But no good reason for a 2.4-6MHz change in linewidth

ã1A1(010) Sub-Doppler measurement Linewidth(FWHM): 2.4 MHz 313 was selected as the control line: unmixed, and similar intensity as the 616 Splitting: 2MHz Splitting: 3MHz Splitting: 1MHz Estimate of mixing coefficient 2%<b2<5%  its triplet partner is too dark to be observed (consistent with observation)

Kinetic signatures of mixed states Time l 616 & 523 have similar decay at longer time however, the decay curve appears to be different during the first few hundreds ns. 523 was selected to be a control line because it is unmixed and has similar rotational energy with 616 Ratio of the FM signal looks constant at the longer time Slope at first 400ns show that 616 has about <5% triplet mixing

How about the para lines? Sub-Doppler measurements can’t be done since it’s para level 515(T) appears very differently with a much slower growth at early time Since the partner of the 515 was identified  Fraction of total mixed state signal in S orT . 515 Mixing coefficient b2 is about 10% of the triplet mixing

Summary New spectrometer was constructed with improvement of saturation dip spectral linewidth Sub-Doppler and kinetics measurement on the ã(010) 616 line suggests a very small mixing coefficient between singlet and triplet states Kinetics measurement of the ã(010) 515 S&T lines show the mixing coefficient, b2 is about 10%. The question remains the same: ã(010) & ã(000) have similar kisc rates, yet search for gateway state finds inadequate strength and abundance of mixed states

Acknowledgements Dr. Gregory Hall Prof. Trevor Sears Group members at BNL U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences Contract No. DE-AC02-98CH10886

V=0 18e

313 line 28w

18h

616 1 line gamma: 3. 28s

616 3 lines fitted fixed gamma:1.44 residue: 0.000586123