Speed dependent rotational angular momentum polarization of the O2 (aΔg1) fragment following ozone photolysis in the wavelength range 248–265nm

Abstract

The translational anisotropy and rotational angular momentum polarization of a selection of rotational states of the O2 (a 1Deltag; v=0) photofragment formed from ozone photolysis at 248, 260, and 265 nm have been determined using the technique of resonance enhanced multiphoton ionization in combination with time of flight mass spectrometry. At 248 nm, the dissociation is well described as impulsive in nature with all rotational states exhibiting similarly large, near-limiting values for the bipolar moments describing their angular momentum alignment and orientation. At 265 nm, however, the angular momentum polarization parameters determined for consecutive odd and even rotational states exhibit clear differences. Studies at the intermediate wavelength of 260 nm strongly suggest that such a difference in the angular momentum polarization is speed dependent and this proposal is consistent with the angular momentum polarization parameters extracted and reported previously for longer photolysis wavelengths [G. Hancock et al., Phys. Chem. Chem. Phys. 5, 5386 (2003); S. J. Horrocks et al., J. Chem. Phys. 126, 044308 (2007)]. The alternation of angular momentum polarization for successive odd and even J states may be a consequence of the different mechanisms leading to the formation of the two O2 (a 1Deltag) Lambda doublets. Specifically, the involvement of out of plane parent rotational motion is proposed as the origin for the observed depolarization for the Delta- relative to the Delta+ state.

Photofragment angular momentum distributions from oriented and aligned polyatomic molecules: beyond the axial recoil limit

“…This formalism, and an equivalent molecular-frame formalism by Rakitzis and Zare [3], was used to successfully describe the photofragment polarization measured from the photodissociation of a number of diatomic molecules (well described by the axial recoil approximation) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. More recently, photofragment polarization has been measured from the photodissociation of polyatomic molecules, for which the axial recoil approximation breaks down [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. The polarization measurements can be fitted with the same polarization parameter basis functions (as they form a complete basis), however the interpretation of the values of the polarization parameters is no longer straightforward or well-understood.…”

Frequency modulated circular dichroism spectroscopy: application to ICN photolysis

“…This velocity dependence was confirmed and characterized for a wide range of N in v = 0, for both 248 and 266 nm photolysis, by Costen and Hall using FM Doppler spectroscopy [7], These experiments demonstrated that single rotational levels of CN produced in the I and I* channels did indeed have opposite orientations, and that both the integral and Doppler-resolved orientations could be described by a single parameter. [38][39][40]. The molecular frame orientation in these systems integrates to zero in the laboratory frame, and is undetectable with one-photon Doppler absorption spectroscopy, although it can be observed with REMPI time-of-flight, Doppler-resolved LIF with polarization analyzed emission, or ion-imaging methods.…”

Origin of the “odd” behavior in the ultraviolet photochemistry of ozone

“…ozone | photochemistry | quantum dynamics | isotopic fractionation | lambda doublets G iven the importance of the stratospheric ozone layer in protecting the earth from harmful ultraviolet (UV) rays (1), it is not surprising that the UV photodissociation of ozone has been the subject of numerous experimental (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) and theoretical studies (16)(17)(18)(19)(20)(21)(22)(23)(24). In the strongly absorbing Hartley band (200-300 nm), the photodissociation involves two low-lying excited states with the same symmetry (A′) as the ground state (X): the B state which diabatically correlates to O 2 (a 1 Δ g ) and O( 1 D) products, and the R state which diabatically correlates to O 2 (X 3 Σ − g ) and O( 3 P) products.…”