ISOTOPICALLY SELECTIVE INFRARED MULTIPLE-PHOTON DISSOCIATION OF MoF 6 BY A H2-RAMAN LASER

Infrared multiple-photon dissociation (IRMPD) of MoF 6 has been investigated with a parallel beam of a para-H 2 Raman laser newly developed by our group. MoF 6 gas (2 Torr,natural abundance) was put into in a static cell cooled to -58C ad was irradiated with the laser line a 728 cm-1 The critical fluence of 0.11 Jcm- for complete dissociation was determined by analysis with a Gaussian beam model. Iradiation at the lower frequency side (728 cm- ]) of the MoF 6 band enriched the lighter isotopes in the residue. A maximum value for selectivity of 1.1 was obtained.


INTRODUCTION
The laser method of isotope separation has many of advantages compared with existing methods for separating isotopes of heavy elements. 0nly a few papers have been published on the laser isotope separation of heavy metals other than uranium. Isotopic enrichment of osmium was investigated by Ambartzumian et al. by IRMPD of OsO 4 with OCS as a scavenger. Tiee and Wittig 2 photolyzed SeF 6 with an NH 3 laser and C0 2 laser and observed that lighter isotopes were dissociated more efficiently than the heavier ones. Freund and Lyman 3 carried out isotopically selective dissociation of MoF 6 by irradiation of weak combination 235 bands with CO 2 laser radiation.
Recently many heavy metal isotopes have attracted much interest as useful materials in the field of atomic energy engineering and nuclear medicine. Molybdenum isotope is an example. Use of 97Mo in the steel of a fusion-reactor first wall is desirable because it can reduce induced long-term radioactivity . Molybdenum-98 is used as a starting material for the mother element 99Mo producing 99mTc in nuclear medicine. Isotope shifts for heavy metals are small due to their high mass number. Furthermore, the hot band problem in heavy molecules is much more severe than in lighter molecules. Only 1.0 of the MoF 6 is in the vibrational ground state at 300K 5 and a large number of thermally populated vibrational states tend to shift and spread spectral features. The isotope shift is obscured by such spectral interference due to hot bands. However, isotope separation can be achieved on the isotope-shifted slope of the Q-branch envelopes by lowering the gas temperature to some extent. In this paper IRMPD of MoF 6 using a para-H 2 Raman laser was carried out and the results were analyzed on the basis of Gaussian beam model. It was observed that molybdenum isotope was enriched in the residual MoF 6. absorption cell (20 mm in diameter, 5 cm in length) which contained 300 Torr of C2HsF, and the residual radiation at 728 cm -I was used for irradiation of MoF 6 in a reaction cell (80 cm in length, 00 ml in volume). The maximum output energy at 728 cm -I was 300mJ with a 50 ns pulse duration at 3 Hz repetition rate. The pulse energy was measured by a pyroelectric detector before and after passing through the reaction cell. The reaction cell had KBr side windows for measurement of infrared absorption. Perflon 0-rings were used for vacuum sealing of the windows. Prior to irradiation, the cell wall was passivated with 600 Torr of 5 F 2 in He for 15 min at 100 C. It was confirmed that there was negligible decomposition of MoF 6 by corrosive action. MoF 6 was purchased from Kantodenka Co. Ltd. and subjected to freeze-pump-thaw cycles before use. The cell was kept at -58C by immersing it in a n-octane slush bath.
The fraction of dissociated MoF 6 was monitored during the irradiation by the measurement of the 3 band absorption at 741 cm -I with an infrared spectrometer. After irradiation, undissociated MoF 6 was recovered from the cell and transferred into a sample bottle immersed in a liquid nitrogen dewer and introduced into a mass spectrometer. The isotope ratios of both irradiated and natural MoF 6 were then determined from the mass spectrum of the MoF5 + isotopic ions.

RESULTS AND DISCUSSION
Infrared muti_l.e,photon dissociation The radial intensity distribution of the laser beam in multi-mode radiation of a TEA-CO 2 laser is relatively homogeneous. However, the Raman laser beam after conversion of single-mode TEA-CO 2 laser radiation is not homogeneous and has a Gaussian transverse profile. Therefore it is necessary to consider the inhomogeneity of the beam in deriving the relation between IRMPD dissociation yield and fluence. According to the analytical model for the reaction yield vs. fluence with an unfocused Gaussian beam under the optically thick condition 7, the critical fluence, @c' for complete dissociation of a molecule is obtained experimentally from measurement of G(O,O), VR, and n. Here @(0,0) is the Gaussian peak fluence at the cell entrance, to is the Gaussian peak width at I/e 2, is the overall absorption coefficient in infrared multiple-photon absorption, V R is the reaction volume, and n is the value for the power dependence of dissociation probability upon fluence.
In the IRMPD of 2 Torr of MoF6, (0,0), oo and were determined from the energy measurement to be 1.60I0 "2 J cm-2 -I 3.19 cm, 2.29I0 -2 cm respectively. The reaction volume, VR, was determined to be 1.02I0 -2 cm 3 by using the following relation b=VR/Vcell (2) where Vcell is the cell volume and b is the specific dissociabtion rate constant obtained from the conversion, X, after irradiation by t pulses.
The value of n was assumed to be , which is typical in most IRMPD experiments 8. The critical fluence, @c' was thus estimated as 0.11Jcm -2 for 2 Torr of MoF 6 cooled at Isotope selectivity The enrichment factor, res' after irradiaton is defined in the case of a multicomponent isotopic system. res=percent of isotope i after irradiation/percent of The enrichment factor, res' depends on the conversion, X.
The conversion-independent selectivity, S, is obtained from res and X as follows. S=1+ln re s/in(I-X) Jcm -2 according to the analytical model. This appreciably low critical fluence strongly suggests that MoF 6 is one of the most promising working substance for the laser isotope separation of molybdenum.