Quantum beat spectroscopy as a probe of angular momentum polarization in chemical processes. Mark Brouard

Transcription

1 Quantum beat spectroscopy as a probe of angular momentum polarization in chemical processes. Mark Brouard The Department of Chemistry Oxford University Gas Kinetics Meeting, Leeds, September 2007

2 The Pilling Legacy Many thanks and happy retirement!

3 Acknowledgements The Group Raluca Cireasa Chris Eyles Yuan-Pin Chang Visiting scientist D.Phil. student D.Phil. student Alessandra la Via Part II student Alistair Green Part II student David Case Part II student Nicholas Screen Part II student Alexander Bryant Part II student

4 Acknowledgements Collaborations F. Javier Aoiz QCT calculations Jaçek K los PES and QM calculations Marcelo P. de Miranda Stereodynamics Funding EPSRC Royal Society

5 Collisional depolarization

6 Collisional depolarization Z J J j j How easy is it to change the direction of J by collision? Relevant to the detection of OH(X) or NO(X) by LIF.

7 Collisional depolarization K Can be characterized in terms of the angular momentum transferred, K Often assumed that K is minimized in collisions

8 Angular distribution (OH(A) + Ar) Increasing K QCT calculations by C.J. Eyles and F.J. Aoiz New PES by J. K los and M.H. Alexander

9 Angular distribution dσ dω jj = σ n (2n + 1) 2 a n P n (cos θ jj ) Disalignment (even terms) a 2 = P 2 (cos θ jj ) 0.5 a Disorientation (odd terms) a 1 = P 1 (cos θ jj ) 1.0 a

10 Motivation Rotational polarization Angular dependence of potential energy surface Mechanistic information Aims Measure polarization using quantum beat spectroscopy. Weak magnetic field effects in chemistry. Control of angular momentum orientation and alignment.

11 Zeeman quantum beat spectroscopy

12 OH source and detection Pump H 2 O 2 + hν OH(X 2 Π) + OH(X 2 Π) Probe OH(X 2 Π) + hν OH(A 2 Σ + ) [ or NO(X 2 Π) + hν NO(A 2 Σ + ) ] Use a long (250 ns or 10 µs) pump-probe laser delay.

13 Experiment Detect OH(X 2 Π) by polarized laser induced fluorescence... Kr...in presence of a weak magnetic field.

14 OH(X) spatial distribution Spatial distribution of OH(X 2 Π) is nearly isotropic. H Z X Y No net magnetic moment, no precession about the field

15 Initial OH(A) spatial distribution Excite OH(X) with linearly polarized probe radiation. Transition probability P ˆµ OH ˆǫ a 2 H Z a X Y Generates an aligned ensemble of excited OH(A 2 Σ + ) radicals.

16 Zeeman quantum beats Precesses in magnetic field with Larmor frequency, ω L. H Z a X L t Y Observe emission through a linear polarizer.

17 Zeeman quantum beats Alternative picture: R 11 (4) transition Coherent excitation of Zeeman levels.

18 Link with theory (linearly polarized light) Initial aligned distribution P(θ j ) = 1 2 [ 1 + A20 P 2 (cos θ j ) ] Distribution after one collision P(θ j ) = 1 2 [ 1 + A20 a 2 P 2 (cos θ j ) ]

19 Collisional depolarization of OH(A) and NO(A) by Ar at 300 K

20 Zeeman quantum beats No field: OH R 11 (4) transition intensity / arb. unit H =0 Gauss time / ns Exponential population decay [OH ] = [OH ] 0 e k 0t

21 Zeeman quantum beats Population decay [OH ] = [OH ] 0 e k 0t k 0 = k rad + k Q [Ar] k rad - radiative decay (τ rad 700ns for OH(A)) k Q - electronic quenching (relatively small for Ar)

22 Zeeman quantum beats With field: R 11 (4) transition 1.0 H = 4 Gauss Intensity / arb. unit Time / ns [OH ] = [OH ] 0 e k 0t {1 + C e k 2 t F cos(2πω L t + φ)}

23 Zeeman quantum beats [OH ] = [OH ] 0 e k 0t {1 + C e k 2t F cos(2πω L t + φ)} with ω L = g F µ 0 H/h Oscillations at two frequencies for F = 5 and 6.

24 Zeeman quantum beats Depolarization and dephasing: Beat amplitude, C [OH ] = [OH ] 0 e k 0t {1 + C e k 2 t F cos(2πω L t + φ)} Intensity / arb. unit Time / ns Proportional to rotational alignment of excited OH(A)

25 Zeeman quantum beats With Field: Pressure dependence Fluorescence Intensity R e la ti ve In tens it y mtorr Ar 250mTorr Ar Time (ns) Time (ns) Collisional population decay and depolarization

26 Zeeman quantum beats Depolarization and dephasing [OH ] = [OH ] 0 e k 0t {1 + C e k 2 t F cos(2πω L t + φ)} k 2 = k inhom + k d [Ar] k inhom - dephasing by field inhomogeneities k d - collisional depolarization by Ar (k d v rel σ d )

27 Link with theory Depolarization rate constant, k d v rel σ d k d = k c (1 a 2 ) where k c is the collision rate constant (e.g., for energy transfer) Three cases: 1. a 2 = +1.0 k d = 0 no depolarization 2. a 2 = 0.0 k d = k c depolarization rate same as collision rate 3. a 2 = 0.5 k d = 1.5k c depolarization faster than the collision rate

28 Zeeman quantum beats Trends in depolarization cross-sections: OH(A) + Ar (300 K) Cross-sections are large (long range interaction). Cross-sections decrease with N (angular momentum conservation).

29 Zeeman quantum beats Collisional processes leading to depolarization Inelastic depolarization (rotational energy transfer) Elastic depolarization (velocity changing)

30 Zeeman quantum beats Comparison with rotational energy transfer: OH(A) + Ar (300 K) Depolarization more efficient than RET (a 2 0 for this system) Elastic contribution to σ d 20 Å 2 for N = 4 E.A. Brinkman and D.R. Crosley J. Chem. Phys. (2004)

31 Zeeman quantum beats Caveat: we detect unresolved OH(A) emission Populated levels have different g F values - leads to a dephasing Important for spin-rotation changing collisions Effects can be accounted for, although better to resolve emission

32 Comparison with hyperfine quantum beats: NO(A) Coherent superposition of hyperfine levels (Low N ) Observe two of the three Hyperfine beat frequencies.

33 Hyperfine quantum beats: NO(A) Initial distribution of J d Z a X Y Nuclear spin, I, initially unpolarized.

34 Hyperfine quantum beats: NO(A) Alignment of J reduced d Z a X Y Nuclear spin, I, becomes aligned.

35 Hyperfine quantum beats: NO(A) Alignment of J and I cycle in time d Z a X Y See T.P. Rakitzis, Phys. Rev. Lett. (2005)

36 Hyperfine quantum beats: NO(A) Beat signal S 21 (0) R 22 (4) Intensity / arb. units Intensity / arb. units Time / ns Time / ns Amplitude decreases rapidly with J.

37 Hyperfine quantum beats: NO(A) Depolarization cross-sections Hyperfine Zeeman NO(A) + Ar (300 K) Reasonable agreement between hyperfine and Zeeman beat data Depolarization is less efficient than RET (a 2 > 0 for NO(A) + Ar)

38 Trends in depolarization cross-sections OH(A) + Ar versus NO(A) + Ar at 300 K NO(A) + Ar OH(A) + Ar Well-depth for NO(A)+Ar is one tenth that of OH(A) + Ar Balanced by kinematic/energetic factors

39 OH(A) + Ar potential Strongly attractive and highly anisotropic PES J. K los and M.H. Alexander, in preparation (2007) R [a 0 ] Θ Well depth 1600cm 1

40 NO(A) + Ar potential Very weakly attractive PES N. Shafizadeh et al., J. Chem. Phys (1998) D 0 44cm 1 T.G. Wright and coworkers, J. Chem. Phys. (2000)

41 Role of electron and nuclear spin Spin is a spectator in 2 Σ + radicals f 2 : J = N S f 1 : J = N + S N N S Spin-rotation changing collisions only occur if N is strongly depolarized.

42 OH(A) + Ar and spin-rotation changing collisions QM QCT Play an important role for OH(A) + Ar QCT calculations by C.J. Eyles and F.J. Aoiz QM and new PES by J. K los and M.H. Alexander

43 OH(A) + Ar and spin-rotation changing collisions Increasing K QCT calculations by C.J. Eyles and F.J. Aoiz New PES by J. K los and M.H. Alexander Spin-rotation changing collisions require large K These are enhanced for OH(A) + Ar by the deep well

44 OH(A) + Ar and rotational energy transfer Disalignment coefficients QCT QCT calculations by C.J. Eyles and F.J. Aoiz New PES by J. K los and M.H. Alexander

45 Final thought Disorientation coefficients? 500 Use circularly polarized light Intensity / arb. unit Time / ns Provides a means of measuring a 1 = P 1 (cos(θ))

46 Zeeman quantum beats Collisional depolarization: Some conclusions. Less efficient at high N - angular momentum conservation. Attractive long-range interaction plays crucial role. Both elastic and inelastic depolarization can be important. Depolarization efficiency relative to RET is very system dependent. For 2 Σ + radicals S and I are spectators in the collision. σ d is large for spin-rotation and hyperfine state-changing collisions.

47 The End