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FIELD IONIZATION OF CH 3 I IN SUPERCRITICAL AR
C. M. Evans 1 and G. L. Findley 2
1 Department of Chemistry and Biochemistry, Queens College – CUNY,
65-30 Kissena Blvd, Flushing, NY 11367
2 Department of Chemistry, University of Louisiana at Monroe,
Monroe, LA 71209
Supercritical fluids have recently been shown to improve rates and modify product ratios
of chemical reactions, to vary chemical shifts in NMR, and to alter lifetimes and energies of
molecular vibrational states. However, the detailed nature of the molecule (i.e., dopant)/fluid
(i.e., perturber) interactions that lead to these effects is not well understood. In recent studies of
the density dependence of solvatochromic shifts of vibrational and UV-visible absorption bands,
an increase in the energy shift near the critical density along the critical isotherm of the perturber
was observed (cf. Fig. 1). In measurements of the field ionization of high-n CH 3 I Rydberg states
in supercritical argon, we observed a decrease in the shift near the critical density along the
critical isotherm (cf. Fig. 2). Such a dramatic difference in behavior is striking. The densitydependent
shift of the dopant ionization energy in dense media can be written as a sum of
contributions, = w 0 (P) + V 0 (P), where w 0 is the shift due to the average polarization of the
perturber by the ionic core, V 0 is the quasi-free electron energy in the perturbing medium and P
is the perturber number density. Our preliminary analysis suggests that while w 0 shifts in a
manner similar to the vibrational and UV-visible band shifts, V 0 does not. Thus, the difference in
behavior between the shift of high-n Rydberg states and of vibrational (or UV-visible) absorption
bands is due to the interaction of the quasi-free electron with the perturbing medium.
This work was conducted at SRC (NSF DMR-0084402) and was supported by a grant from
the Louisiana Board of Regents Support Fund (LEQSF (1997-00)-RD-A-14).
1996
0.0
1994
-0.2
line position (cm )
-1
1992
1990
1988
(eV)
-0.4
-0.6
1986
-0.8
1984
0 2 4 6 8 10 12 14 16 18
density (mol/L)
Fig. 1. Infrared absorption line peak position of the T 1u
asymmetric CO stretching mode of W(CO) 6 in CO 2 vs
CO 2 density at (!) the critical temperature of 33C and
() 50C. The lines provide a visual aid. (Modified
from R.S. Urdahl, D. J. Myers, K. D. Rector, P. H.
Davis, B. J. Cherayil and M. D. Fayer, J. Chem. Phys
107, 3747 (1997).)
-1.0
0
5
10
15
Ar
(10 21 cm -3 )
Fig. 2. Ionization potential of CH 3 I in argon
plotted as a function of argon number density Ar.
() -114C; () -118C; () various lower
temperatures other than the critical temperature;
() the critical temperature of -122C. The lines
provide a visual aid. (C. M. Evans and G. L.
Findley, to be published.)
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