4A05
SO2 紫外線スペクトルにおける圧力及び同位体効果（上智大理工 1，東工大・惑星 2，コペンハゲン大 3）○Sebastian Danielache1，遠藤美朗 2、鈴木 智也 1、上野雄一郎 2、南部 伸孝 1、Matthew S. Johnson３

Pressure and Isotopic effects on the ultraviolet Spectra of SO2(Sophia Univ. 1, Tokyo Tech. 2, Copenhagen Univ.3 ) ○S. O. Danielache, Y. Endou2, Y.
Ueno2, S. Nanbu1, M Johnson3
Introduction.
Most chemical reactions produce mass dependent isotopic fractionation while Sulfur
Non-Mass Dependent (NMD) fractionation signals reported (Fig.1) for the Archean and Early
Proterozoic (>2300 Ma). The photodissociation of sulfur bearing species may have played asignificant role since the concentration of oxygen is estimated to be 10-5 times present
atmospheric levels and therefore ultraviolet light permeates throughout the entire
atmosphere, however the underlying mechanisms are not fully understood. Understanding
the mechanism of sulfur isotopic fractionation during the photodissociation of SO2 in reducing
atmospheres is important for interpreting the geological record describing the rise of oxygen.
In this report we present newly obtained data sets from experimental measurements andtheoretical calculations. From the comparison of theoretical and experimental dates wediscuss the limit the spectral resolution needed to obtain a fully resolved spectrum and also
hap atmospheric pressures effects affect isotopic effects during the photodissociation reaction.

Fig. １ NMD signal preserved in the geological record (Fig. adopted fromMasterson et al.1)

Experimental.
We used a dual beam monochromator in order to obtain higher accuracy cross sectionthat is complementary to high spectral resolution Fourier transform spectrometer andpreviously reported [1]. High-precision and high-accuracy data of the ultraviolet absorption
cross sections of 32SO2, 33SO2, 34SO2 and 36SO2, recorded 190 to 225nm at room temperature,with slit width of 0.1nm, data interval of 0.02nm. SO 2 samples were produced in an identicalprocess via combustion of isotopically enriched S 0, eliminating effects due to variation inoxygen isotopic composition. Our measurements show red shifted peak position for heavier
isotopologue relative to the 32SO2, though rotational structures cannot be observed due to low
spectral resolution we use these spectra to combine the analysis with theoretical calculations.

Theoretical.
In the present work, we performed a theoretical analysis of photolytic isotopic
fractionation factors with a first principle simulation. The ultraviolet absorption crosssections were computed using the wave packet propagation technique to explore the influenceof excited state dynamics, transition dipole surface and initial vibrational state. The
wave-packet propagation technique used here was the “real wave packet” mehod2. Jacobi
coordinates were employed to describe the relative position of the three nuclei in the body
fixed plane. The wave packet is represented using evenly spaced Fourier grids for R and rcoordinates, and associated Lgendre polynominals based discrete variable representation(DVR) for the angle . We calculated the vibrational wave functions of the ground state using
the Lanczos method. After constructing the initial wave packet, the time evolution on several
PES were calculated and the autocorrelation function was computed by numericallyintegrating the product of the initial wave packet and the wave-packet at each time step.
Total photo-dissociation cross section as a function of the excitation energy was computed as
the Fourier transform of the autocorrelation function as a function of time. We compare
calculated spectra for each isotopologue with experimental data and present an attempt toconstruct a rotationally resolved spectra.

References.
[1] ]J. F. Kasting, SCIENCE, 293, 819-820 (2001)[2] Gray and Balint-Kurti, J Chem. Phys., 108, 950 (1998).