Circularly polarized excitation

Lineaxly polarized excitation Circularly polarized excitation ... 

Figure 9.29 Two-photon fluorescence excitation spectrum of 1,4-difluorobenzene. The upper and lower traces are obtained with plane and circularly polarized radiation, respectively, but the differences are not considered here. (Reproduced, with permission, Ifom Robey, M. J. and Schlag, E. W., Chem. Phys., 30, 9, 1978)...

Direct probe of ligand field and charge transfer excited states Greater sensitivity than ABS in observing weak transitions and greater resolution due to differences in circular polarization complimentary selection rules aiding in assignment of electronic transitions... 

Photoluminescence (PL) in the polysilanes is well documented,34b,34c and for the poly(diarylsilane)s occurs typically with a small Stokes shift and almost mirror image profile of the UV absorption.59 This is due to the similarity of the chromophore and fluorophore structures in the ground and excited states, respectively, which is a result of the fact that little structural change occurs on excitation of the electrons from the a to the a orbitals. As PL is the emissive counterpart to UV, the emissive counterpart to CD is circularly polarized pho-toluminescence (CPPL). Where the fluorophore is chiral, then the photoexcited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/R and /l, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality... 

D. H. Metcalf, S. W. Snyder, S. Wu, G. L. Hilmes, J. P. Riehl, J. N. Demas, and F. S. Richardson, Excited-state chiral discrimination observed by time-resolved circular polarized luminescence measurements, /. Am, Chem. Soc. 11, 3082-3083 (1989). 

Developments in instrumentation have allowed measurements of the single-crystal CD spectra of [(+ )D-Cr(en)3]3+ doped in [Ir(en)3]Cl3 between 7 and 293 K. Transitions to the excited states 4T2, 47i, ZE, 27i and 2T2 were observed.418 The discussion refers to much earlier work. The polarized electronic spectra of single crystals of several trans-Cr [XY(en)2] complexes have been assigned and ligand field parameters evaluated.419 CD, absorption and circularly polarized emission spectra have been reported for [Cr(en)3]3+ in the region of the A2g 2EgTig transitions.420... 

I think it is possible to excite either a clockwise or an anticlockwise pseudorotation state by using a circularly polarized beam and also to probe with a circularly polarized beam to detect the circular dichro-ism induced by the pump beam. 

While ionization by linearly polarized fields has been well studied, there is only one report of ionization by a circularly polarized field, the ionization of Na by an 8.5 GHz field.36 In the experiment Na atoms in an atomic beam pass through a Fabry-Perot microwave cavity, where they are excited to a Rydberg state using two pulsed tunable dye lasers tuned to the 3s — 3p and 3p —> Rydberg transitions at 5890 A and —4140 A respectively. The atoms are excited to the Rydberg states in the presence of the circularly polarized microwave field which is turned off 1 fis after the laser pulses. Immediately afterwards a pulsed field is applied to the atoms to drive any ions produced by microwave ionization to a microchannel plate detector. To measure the ionization threshold field the ion current is measured as the microwave power is varied. 

Fig. 10.21 Ionization threshold fields for linear (+) and circular ( ) polarization as a function of n when Na atoms are excited in the 8.5 GHz microwave field (from ref. 36).

Schippers and Dekkers reported on the CD and circularly polarized fluorescence of 4,4-dideuterio-adamantan-2-one (85)193. The CD of 85 originates in transitions to a totally symmetric n -+ n excited state with double minimum potential in the C=0 out-of-plane bending mode. 

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