Reversibility of intersystem crossing in the ã 1 A 1 (000) and ã 1 A 1 (010) states of methylene, CH 2 ANH T. LE, TREVOR SEARS a, GREGORY HALL Department of Chemistry Brookhaven National Laboratory Upton, NY, USA a. Department of Chemistry, Stony Brook University, Stony Brook, New York International Symposium on Molecular Spectroscopy 70th Meeting, June 22-26, 2015 Champaign-Urbana, Illinois TI13

k i S T ‡ k S k -i k T Products Relaxed T ISC Reverse ISC What is reverse intersystem crossing (ISC) ? CH 2 bending potential a state X state |S> |T> =0 =1 =2 =3   S-T = 3156(5) cm -1, CH 2 made in singlet; good chance that triplet will be populated by collision  Only a small number of rotational states of the isolated CH 2 molecule have a mixed singlet/triplet electronic character.  These special mixed states are presumed to act as gateways between the otherwise unconnected S and T manifolds CH 2, provides a unique model case for investigating collisional induced (ISC) transfer in small polyatomic at a fully rotation-vibration quantum state resolved level of detail

Previous work on reversibility of ISC Kinetics of the (para) 5 15 state measured at a single frequency in a sample of 4% ketene in Ar at a total pressure of 1.25 Torr. Double exponential decays are observed for all rotational levels of vibrationless CH 2 (ã 1 A 1 ), a direct kinetic consequence of reversible intersystem crossing (unpublished work) I(t) time dependent intensity A amplitude, s relative amplitude of the slow decay component k f & k s Much works has been reported on CH 2 Photolysis by Excimer laser

The slow decay rates are the same for ortho and para. The relative amplitude of the slow decay component, s, is always larger for para than for ortho CH 2 and increases with high dilution of ketene in rare gas, approaching ~2% for ortho and ~8% for para CH­ 2. Previous work on reversibility of ISC (cont.) Slow fast One more collision partner, Quenching kinetics of singlet CH 2 in samples of precursor, Ar and O 2 ( 3  g - )? 330mTorr 10% ketene/Argon premixed with 60, 120, 180 mTorr of O 2, measurements were done on v=0: 2 12, 2 11, 7 17, 6 16, 7 16 (S), v=1: 3 13,4 14, 5 15 (S)

Integrated population Repeat for each time slice from beginning to the end of the time domain Non thermal fit FM signal Time (20ns/Step)

v=0, 6 16 ortho v=0, 7 17 para Results v=0 The initial decay rate of selected rotational states of CH 2 is accelerated by O 2 The relative amplitude of the slow decay component, larger for para than for ortho CH 2, and enhanced by O 2  a continued role for reversible population of vibrationally excited triplet CH 2 in the overall mechanism.

Integrated population on v=0 were fitted to equation: StateLine removal Rate constant by Oxygen(/10 11 ) cm 3 /(molecules.s) v= (2) (4) (8) (8) (8) ref5.2 a ; 0.4 b ;2.7 c ;3.0 d a. M.A. Blitz et al. / Chemical Physics Letters 372 (2003) 295–299 b. Langford et al, J. Chem. Phys. 78 (1983) c. Ashfold et al., J. Photochem. 12 (1980) 75. d. Ashfold et al., Chem. Phys. 55 (1981) 245. The ratio of slow to fast amplitudes, s, with the partial pressure of Oxygen Variation of fast and slow decay rates, k f and k s

Results v=1  No obvious sign of double exponential decay in v=1  Fit to single exponential decay  3 13,4 14,5 15 (S) show similar removal rate by Oxygen (~3.6x cm 3 /(molecules.s) v=1, 3 13 Variation of decay rates in =1

Kinetic modeling studies Double exponential decays in no Oxygen data: Single exponential decay Using Energy grained master equation (EGME) with exponential down energy gap model for rotational energy transfer within each vibronic level  Probability of downward transfer depends only on the energy difference between two levels, and the average energy transfer per collision,  E d  The probability of upward transfer is determined by microscopic reversibility and ensures that the long time solution in the absence of reaction is the Boltzmann distribution. Time dependence of population density in energy grain i is described by  collision frequency Pij: conditional probability of transfer from bin j to bin i k i : is the effective rate of decay out of vibrational level. If no mixed states, singlet and triplet manifolds are totally uncoupled Un-connected, Zero elements in off-diagonal matrix.

Log plot – time dependence of population density  The row entries represent transfer from other pure singlet levels into the mixed state  The column entries represent collisions out of the mixed state that terminate in a non-mixed triplet state. To adjust for loss out of the mixed singlet level, these terms are subtracted from the diagonal component. Mixed states double exponential decays Non-Zero elements in off-diagonal matrix. Double exponential decays

Double exponential decays in Oxygen data Increasing in both k f & k s by adding O 2 suggested that O 2 plays important role in whole mechanism How O 2 will be entered into EGME of whole system Will require additional parameters that included rotational relaxation rate for O 2, reaction with O 2 for singlet, reaction for triplet… Work is in progress Challenges: 300mTorr of argon, 30 mTorr ketene Variation of fast and slow decay rates, k f and k s The ratio of slow to fast amplitudes, s, with the partial pressure of Oxygen

Future work  New experiment in the mid-IR using quantum cascade laser (QCL) is under construction to directly detect vibrationally excited triplet X 3 B 1 Summary CH 2 (ã 1 A 1 )(010)  Much weaker signal  Did not observe any indication of double exponential decays for v 2 =1 states with and without O 2  not enough population for reversibility of ICS in (ã 1 A 1 )(010) CH 2 (ã 1 A 1 )(000)  Double exponential decays are observed for all rotational levels  evidence of reversible intersystem crossing  The relative amplitude of the slow decay component, larger for para than for ortho CH 2, and enhanced by O 2  Need to work on kinetics models to add Oxygen

Acknowledgements Contract No. DE-AC02-98CH10886 and DE-SC Dr. Gregory Hall Prof. Trevor Sears Group members at BNL Thank you

Experimental set-up Key of this experiment is how to control the amount of oxygen inside the cell: 1.10% 330mTorr ketene/Argon premixed. 2.60, 120, 180 mTorr of O 2 were added to the sample cell in a controlled way 3.Repetition rates were decreased as the amount of oxygen increased to make sure that the ketene sample is fresh. 4.Measurements were done on selected rotational lines: v=0: 2 12, 2 11, 7 17, 6 16, 7 16 (S), v=1: 3 13,4 14, 5 15 (S)