Atomic, Molecular and Optical Physics Laboratory______________________________ Collisional Depolarization of Zeeman Coherences in the 133 Cs 6p 2 P 3/2 level Burçin Bayram Physics Department Miami University Oxford, OH

Atomic, Molecular and Optical Physics Laboratory______________________________ Two-photon Polarization Spectroscopy Group Members: Undergraduate Students: Morgan Welsh, Jacob Hinkle Graduate Students: Seda Kin, Ramesh Marhatta Dr. Burçin Bayram Advisor: Dr. Burçin Bayram Collaborations: - Mark Havey, Old Dominion University, Physics Dept., VA -Andrei Sieradzan, Central Michigan University, Physics Dept., MI -Marianna Safronova, University of Delaware, Phys. Dept., DE Funding: -Research Corporation -Miami University, Oxford, Ohio

Atomic, Molecular and Optical Physics Laboratory______________________________ focuses on the fundamental properties of atomic/molecular systems using a novel nanoseconds pulse laser spectroscopic technique investigates the evaluation of rotational angular momentum and collisional dynamics between atoms/molecules in the gas phase. Information about the alignment-dependent collisional depolarization cross section – property of great potential for combustion diagnostics, gas phase chemistry, etc. Significance of Polarization Spectroscopy

Atomic, Molecular and Optical Physics Laboratory______________________________ Polarization Spectroscopy (nonlinear coherent technique) can be applied to: -Detection of trace constituents, flame and plasma analysis, concentration measurements and in various atmospheric and combustion processes. What does polarization spectra reflects? It reflects the distribution of the molecular/atomic population on the electronic level of transition. Because of the polarization state of light, excited state become very sensitive to any alignment and/or orientation of the product angular momenta. Rotational Polarization: in molecular product: angular momentum is rotational -Angular dependence of potential surfaces Electronic Polarization in atomic product: angular momentum is electronic; polarization reveals information about electron density distribution function (EDF). -Atmospherically important process Information: temperature, collisional depolarization

Atomic, Molecular and Optical Physics Laboratory______________________________ Why collisional depolarization? -Measure degree of polarization -Evaluation of the angular momenta (radicals, atoms, molecules) - Collisional evaluation of the alignment tensor moments -Relevant to the detection of radicals in flames: measurement of absolute concentrations of radicals require knowledge of collisional depolarization. Two techniques can be applied to study the collisional depolarization and alignment in atomic collisions; Optical excitation of atoms in a beam Optical excitation during the collision itself

Atomic, Molecular and Optical Physics Laboratory______________________________ Partial Energy level diagram and illustration of the experimental scheme 9d 2 D 5/2 9d 2 D 3/2 9p 2 P 3/2 6p 2 P 3/2 6s 2 S 1/ Energy (10 4 cm -1 ) nm nm Ionization Energy: 3.89 eV 10s 2 P 1/2 ~584.5 nm

Atomic, Molecular and Optical Physics Laboratory______________________________ Kastler Diagram for the Cs 6s 2 S 1/2 - 6p 2 P 3/2 – 9d 2 D 5/2 +1/2 +3/2 -1/2 -3/2 -5/25/2 6s 2 S 1/2 9d 2 D 5/2 6p 2 P 3/2 Δm= 0 Δm= ±1 Δm= 0 +1/2 Axially symmetric electronic-alignment component (information of the spatial distribution of angular momentum vector J) is produced by excitation of the 6s 2 P 1/2 - 6p 2 P 3/2 state: pump laser probe laser

Atomic, Molecular and Optical Physics Laboratory______________________________ z y kk Laser 2 Laser 1  E E x 2 E 1  E 2,  = 90 o E 1 // E 2,  = 0 o interaction region Experimental Geometry & General Concept detector

Atomic, Molecular and Optical Physics Laboratory______________________________ Basic Formulas for Data Analysis Compare with Experimental Data Electronic alignment at t = 0 Time evolution of I and J h 2( J,J’):ratio of Racah coefficients that is derived from the reduced matrix elements of the density matrices and depend on the angular momenta of the initial J and final J’ levels. Reference: C.H. Greene and R.N. Zare, Ann. Rev. Phys. Chem., vol.33, 119 (1982 ).

Atomic, Molecular and Optical Physics Laboratory______________________________ Signal Detection 6s 2 S 1/2 6p 2 P 3/2 I 1z I 2z, I 2x 9p 2 P 3/2 S // (j'), S  (j') S // (j') = I // (j') I(10p 6s) where I // (j') = I 1z I 2z S  (j') = I  (j') I(10p 6s) where I  (j') = I 1z I 2x 10s 2 S 1/2

Atomic, Molecular and Optical Physics Laboratory______________________________ HV Polarizer Dye Laser 1 Dye Laser 2 Nd:YAG Pulse Laser 532 nm Filter Boxcar Atomic, Molecular and Optical Physics Laboratory______________________________ PMTPMT Experimental Apparatus LC Retarder Amplifie r frequency controller Computer nm nm

Atomic, Molecular and Optical Physics Laboratory______________________________ Littman-Metcalf Configuration Design of tunable pulsed dye laser cavity Θ Dye Cell GratingDesired Wavelength Output Coupler Mirror Nd:YAG 532nm ~0.2 W Nd:YAG repetition rate: 20Hz Pulse width: 6-7ns

Atomic, Molecular and Optical Physics Laboratory______________________________ Typical Scans Linear polarization spectra at 70 o C. 6s 2 S 1/2 -6p 2 P 3/2 -9d 2 D 5/ cm -1 6s 2 S 1/2 -6p 2 P 3/2 - 10s 2 S 1/2

Atomic, Molecular and Optical Physics Laboratory______________________________ decoupling radius RcRc  zczc perturber path degree of depolarization depends on the duration of the collision Reorientation of the atomic dipole results in a reduced observed polarization when measured with respect to the original excitation z-axis. coupling radius Modification of atomic collisional dynamics Ar polarized Cs (p-Orbital)

Atomic, Molecular and Optical Physics Laboratory______________________________ Rate equation analysis of Zeeman coherences and depolarization cross section The variety of possible distributions of atoms in the Zeeman sublevels depends on the given experimental conditions such as optical pumping with a circularly or linearly polarized light source. Population mixing among the Zeeman coherences.

Atomic, Molecular and Optical Physics Laboratory______________________________ Polarization spectroscopy as a function of pump-probe delay time Polarization degree for I=7/2, J=3/2, h (2) = 0.2 Theoretical Expected

Atomic, Molecular and Optical Physics Laboratory______________________________ Linear Polarization Degree = -0.8 **Havey et. Al, J. Chem. Phys., 86, 1648 (1987) * Greene et al, Ann. Rev. Phys. Chem., 33, 119 (1982) PLPLPLPL g (2) =1 (w/o hpf) g (2) =0.219 Theory** h (2) (J i, J f )Measured 9d 2 D 5/ %3.25 %-1/43.3(1) % 10s 2 S 3/ %15.57 %-4/515.6(3) %

Atomic, Molecular and Optical Physics Laboratory______________________________ Linear Polarization degree of Cs with Argon

Atomic, Molecular and Optical Physics Laboratory______________________________ Nonlinear least-square fit of the polarization spectrum

Atomic, Molecular and Optical Physics Laboratory______________________________ Pressure dependence of the signal where and k d : disalignment rate coefficient P: buffer gas pressure kT: thermal energy constant σ d : alignment dependent cross section

Atomic, Molecular and Optical Physics Laboratory______________________________ References: [4] J. Guiry and L. Krause, Phys. Rev. A 14, [5] A.I. Okunevich and V.I. Perel, Soviet Physics JETP 31, 356 (1970). [16] Havey et. Al, J. Chem. Phys., 86, 1648 (1987) Bayram et al, Phys. Rev A 73, (2006) σ d (Å 2 )g (2) References 186(58)0.219(44)This work (6S-10S) 0.222(20)This work (6S-9D) 0.219(10)Ref Ref (72)Ref. 4 The depolarization cross section of the excited electronic state of cesium atom

Atomic, Molecular and Optical Physics Laboratory______________________________ Calculated and measured the alignment tensor moments in the excited state of cesium atoms Calculated and measured the effects of nuclear hyperfine depolarization Measured the linear polarization spectrum during the collisions between cesium and argon atoms Collisional dependence of the linear polarization spectra is found to decrease significantly with a decreasing Ar pressure Extracted the alignment-dependent collisional depolarization cross section from the spectra Main future goal: apply polarization spectroscopy using two-photon pump-probe technique to a system of colliding radicals with various pressures of He,Ne,Kr,Xe: great potential for combustion diagnostics. Conclusions and Future Directions

Atomic, Molecular and Optical Physics Laboratory______________________________ A new design for continuous variation of noble gas pressure Noble gas in Pump out Cell-oven

Atomic, Molecular and Optical Physics Laboratory______________________________ A view from our laboratory at Miami University, Physics Department Thank you for your attention Relationship with Ohio Third Frontier Project: The overall goal of our research is to promote fundamental research leading to a better understanding of the atomic/molecular dynamics (evaluation of rotational angular momentum and atomic/molecular collisions) in the gas phase. This research has direct application in the areas of gas phase chemistry such as combustion, plasma, and Atmospheric chemistry. Collaborations with other institutions/laboratories in Ohio state can contribute boosting the economy of the State of Ohio.