4406
NOTES
Energy Transfer between Molecules and Electronically Excited At0rns.l
that of the additive M is a t least lo3. Application of the steady-state kinetics to the excited species of argon in such a system would yield the following Stern and Volmer type equation for the change in the rate of ion formation under saturation current measurements4
I1
by M. M. Shahin and S. R. Lipsky Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut (Received July 8, 1966)
Recent investigation9 on the nature of the excited species of argon formed through the action of p-radiation and the mechanisms of energy transfer from these species to various organic and inorganic molecules have provided a clearer understanding of experiments in rare gas sensitized radiolysis of organic compounds. We wish to report some relative values of rate constants for total quenching of these excited species and the efficiencies by which the organic molecules undergo ionization in the process of collision when the transferred energy exceeds their ionization potential. Excited species of argon were formed by the action of tritium prays on argon gas flowing continuously through a microionization chamber3 a t atmospheric pressure. The addition of various quantities of organic vapors (1:103) caused changes in the degree of ionization which were measured by the change in the current collected in the ionization chamber. Following the treatment of Jesse4and othersj2we can write
Ar -+=
Ar+ and subsequently Arz+
where AR is the increase in the rate of formation of ions through reaction 4 when the organic compound M is added to a system of pure argon; K 2 is equal to k2
I
I
I
I
I
I
I
/
BENZENE)
(1)
kz
Ar ---+ Ar*
+ Ar -+ kd
Ar*
(2)
2Ar (all self-quenching reactions)
M+ ki
Ar*
+ e + Ar
(3)
+ Ar
+
The Journal of Physical Chemistry
I
I
I
2.0
3.0
4.0
+
1
5.0
10-3
(4)
(5)
(A) and C is the ratio of the concentration of the organic molecule Lo that of argon present in the chamber. A plot of l/AR as meawured by the current increment in the ionization chamber vs. l / C will yield a straight
Various values of k are the rate constants of the respective reactions; ka describes all the steps through which the excited argon species will be destroyed in pure argon2a and Ar" refers to the electronically excited state of argon2 with an energy of about 11.8 e.v. No higher excited state@ of argon Ar"" (14-15 e.v.) are expected to participate in the ionization act as their high rate constant for reaction with argon atoms Ar, Ar2+ e, k N c~./sec.)~ is expected (Ar** to make any competition for the organic molecules most unlikely since the relative concentration of argon to
+
I.o
Figure 1. Variation of the inverse of the current increment us. the inverse of the relative concentration of the organic vapor to that of argon. Saturation current measurements were made a t 300 v./cm.
+M
molecular decomposition
I
0
f
\bk
I
(1) This work was supported by grants from the National Aeronautics and Space Administration (NsG 192-61) and the National Institute of HeaIth (HE-03558). (2) (a) M. M. Shahin and S. R. Lipsky, J . Chem. Phys., 41, 2021 (1964); (b) G. 8. Hurst, T. E. Bortner, and R. E. Click, {bid.,42, 713 (1965). (3) M. M. Shahin and S. R. Lipsky, Anal. Chem., 35, 467 (1963). (4) W. P. Jesse and J. Sadauskis, Phys. Rev., 100, 1755 (1955). (5) P. M. Becker and F. W. Lampe, J . Chem. Phys., 42, 3857 (1965)'
NOTES
4407
line from which the relative values of the various parameters can be evaluated. Details of the experiments have been reported elsewhere.2a Care has been taken to ensure that no excitation of argon gas occurs through acceleration of the electron in the field and corrections have been applied to the measured current increment in order to remove the effect of the change in electron drift velocity as the organic molecule is introduced into the system. Gas mixtures were all prepared and analyzed by mass spectrometry and further diluted to the required concentrations. The results for a number of gases are shown in Figure 1 and their relative rate constants as measured through the slopes and intercepts of the lines are tabulated in Table I. The over-all accuracy of the tabulated values is not considered to be better than 30%, owing to the accumulated errors including that involved in the extrapolation of the best drawn line through the data for intercept measurements. The relative values are regarded, however, to be more accurate.
Table I: Relative Rate Constants for Energy Transfer from Excited Argon Atoms to Various Organic Molecules
Acetylene Methylacetylene Ethylene Propane n-Butane Neopentane Acetone Benzene
1.25 1.56 2.00 2.06 2.27 1.69 2.10 1.0
1.00 1.09 0.45 0.36 0.61 0.55 0.48 0.42
1.00 0.87 0.28 0.22 0.33 0.41 0.29 0.53
From the results in Table I, it is evident that the quenching of the excited species, although roughly similar for most of the gases, is not reflected in the ionization of the molecules. These results appear ~ have evaluated different from thoseof Hurst, et U Z . , ~ who these rate constant ratios through computer calculations of their data which covered up to 100% of the organic gas, but by no means at the extremely low concentrations at which present measurements were made. The only other data available in the literature are those due t o Jesse4for acetylene and ethylene which if treated in this manner give the rate constant ratios (kl kg)/kd of 1.5 X lo3 and 2.6 X loa, respectively. These values are similar to our 1.25 X loa and 2.0 X lo3 and contrast with 8.5 X lo3 and 18.3 X lo3 as deduced from the results of Hurst, et a1.2b
+
Enthalpy of Solid Solution for a Metastable Silver-Copper Alloy1*
by Ronald K. Lindelb W . M . Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, California (Received July 8, 1966)
Prior experimental work on the enthalpy of solid solution of Ag-Cu alloys has been confined to the narrow limits of compositionswhich exist as equilibrium solid solutions at elevated temperatures. A liquidquenching technique described in another paper3 has made possible the acquisition of data at a composition considerably beyond the limits of solid solubility which exist in the equilibrium phase diagram.4 By this technique suitable foils of single-phase metastable 75.0 atomic % Ag-Cu solid solutions were prepared (from Ag of purity 299.99% and Cu of purity >99.999%) and checked for single-phase composition (using X-ray diffraction). Half of the foils were retained in the metastable condition, while half were heated at 205" in an argon atmosphere for about 200 hr. and were checked by X-ray diffraction to ensure transformation to the stable state (50.2 atomic % solute in solution4). Foils were then cleaned (to remove oxides, etc.) by swabbing with a 28.46 wt. % HN03 solution, rinsed with distilled water, then swabbed with acetone, and allowed to dry in air. The calorimeter consisted of a small, well-insulated double-walled dewar flask provided with a tightfitting cover containing a small inlet door for introducing the specimen. For each experiment, a solution of was added to the calorimeter, 28.46 wt. % "03 which was maintained at 23.0 f 0.5" but which was held constant to within 0.01" during any given experiment. The foils were weighed into sample lots of 0.300 g. each and introduced into the acid solution after the temperature of the system had stabilized. The low rate of heat loss from the system made it possible to wait for all stirring to occur by natural convection currents. The rise in temperature when
(l! (a) This work was sponsored by the U. S. Atomic Energy Commission; (b) Poulter Laboratories, Stanford Research Institute, Menlo Park, Calif. (2) (a) N. Swindells and C. Sykes, Proc. Roy. Sac. (London), A168, 237 (1938); (b) R. A. Oriani and W. K. Murphy, J. Phys. Chem., 62, 199 (1958). (3) R. K. Linde, Trans. A I M E , in press. (4) M. Hansen, "Constitution of Binary Alloys," 2nd Ed., McGrawHill Book Co., Inc., New York, N. Y., 1958, pp. 18-20.
Volume 69, Number 12 December 1966
