RELATIVECROSSSECTIONS FOR
THE
4289
QUENCHING OF Hg(’P1) ATOMS
Table 11), though not susceptible to simple interpretation, does indicate that (as originally suggested2)
ionic processes also play a role in the catalytic contribution t o GG-+$.
Relative Cross Sections for the Quenching of Hg(1PJ Al~ornsl by Albrecht Granzow, Morton Z. Hoffman, and Norman N. Licbtin Department of Chemistry, Boston Unizersity, Boston, Massachusetts
ORB16
(Received June 4 , 1,989)
Resonance emission at h 1849 A was measured at right angles to a X 1849 source as a function of quenching gas pressure in a system containing Hg vapor at room temperature. The emission intensity varied with pressure in accordance with the Stern-Volmer relationship. Cross sections for the quenching of Hg(lPL) relative to Nz mere calculated from the relative slopes of the Stern-Volmer plots for 12 gases (includingNe and Ar) with molecular weights ranging from 2 to 200. The relative quenching cross sections fell jn the range 0.2-3.0 and generally increased with increasing molecular weight. Quenching by He was not detectable. The relatively small differences in these values stand in contrast to the much larger differences in quenching cross w$3ons for Hg(3P1). A number of relative quenching cro9s sections at X 2537 were measured in the same apparatus and agreed with literature values. Introduction Although there is substantial uncertainty in the absolute values of the cross sections for the quenching of Hg(3P1)to the 3P0or ‘So states, relative cross sections are known quite precisely for a large number of quenching gases.2 The rare gases have very low cross sections. The quenching efficiencies of other gases vary over a wide range and depend drastically on molecular structure. Correlations between magnitude of the cross section and availability of vibrational energy levels have not been successful and there appears to be no a priori way of predicting whether quenching of the €IgpP1) will occur to the 3P0or ‘So state. In contrast, data on the quenching of Hg(’P1) are extremely sparse. &Tori3examined the €Ig(‘Pl)-photosensitized decomposition of COS and, by assuming a simple mechanism, calculated an apparent quenching cross section, the value of which depended upon the choice of the uncertain value of the imprisonment lifetime of the ‘PI state. Whatever the absolute value of the quenching cross section, it was recognized that it was larger than the corresponding value for Hg(3P1). Ruland and Perte14 came to a similar conclusion for ethylene. The Hg(’Pl)-photosensitization of hydrocarbons has led to some estimates of relative quenching cross sections. The quenching cross section of isobutane is reported5to be 1.6 times that of propane while the ratio with Hg(3P1) is 3.1.2 The quenching efficiency of cyclobutane is 1.9 times that of cyclobutane-cZ8 compared to the value of 3.8 at 2537 A.6 We have already reported7 that N2 and CO are equally efficient
quenchers of X 1849 8 regonance fluorescence while quenching by He is not detectable and is probably less than one-tenth as efficient. In coptrast, CO is 21.5 times as efficient as Nz in quenching Hg{3P1).2 The data all indicate that energy transfer from Hg(lP1) is less selective than from Rg;(3P1). This paper reports the extendjion of the measurement of the quenching of X 1849 A re$ouPce fluorescence to a total of thirteen cases. Experimental Section The details of the apparatus have already been de~ c r i b e d . ~Incident X 1849 radiqtion from a 1ow-pre:sure Hg resonance lamp was isolated from the X 2537 A line by means of a 7-irradiated &iF filter and impinged upon the fluorescence cell containiog a small quantity of liquid Hg and a plane quartz window at right angles to the incident beam. The intensity of the X 1849 8 resonance fluorescence was measured using a monochromator and photocell with oscilloacopic readout.
4
(1) Supported by the Air Force Office of Scientific Research under Contract AF-AFOSR-765-67. Presented t o the Division of Physical Chemistry at the 156th National Meeting of the American Chemical Society, Atlantic City, N. J., $ept 1W8,Abatreot PHYS 133. (2) J. G. Calvert and J. H. Pit&, Jr., “Photochemistry,” John Wiley and Sons, Inc., New York, N. Y., 1866, pp 74-77. (3) Y. Mori, Bull. Chem BOG.Jap., 34, 1128 (1961). (4) N. L. Ruland and R. Psrtel, J. Anzer. Chem. Soc., 87, 4213 (1965). ( 5 ) R. A. Holroyd and T. E. Fierce, J . Phya. Chem., 68, 1392 (1964). (6) E. G. Spittler and G. W. Weip, ibid,, 72, 1482 (1968). (7) A. Granzow, M. Z. EIoSfman, N. N, Licbtia, and S. K. Wason, ibid., 72,3741 (1968). Volume 73, Numbev 12 December lt9S9
A. GRANZOW, n1.Z. HOFFMAN, AND N. N. LICHTIN
4290 The region between the window of the cell and the slit of the monochromator and the monochromator itself were continuously purged with dry Nz. The total light path through the fluorescence cell was 1.5 cm. All experiments were performed at room ttmperature (23"). Comparative experiments at X 2537 A were performed using a No. 7910 Corning filter in place of the LiF filter. A number of measurements were performed employing a fluorescence cell with 180" geometry and a total light path of 9.5 em. Linde high-purity N2 was further purified as previously de~cribed.~Distilled water, Matheson NH3 and X20, Airco Dry Ice, and Fisher Spectroscopic Grade c-CeH12 were subjected to three freeze (- 196')pump-thaw cycles. Airco 99.95'3, Hz, Matheson Research Grade He, Ne, CH?, CzH4,and CO, Linde prepurified Ar, and K and K Laboratories c-C4Fs were used without further purification. Pure Lab of America high-purity grade Hg was used.
Data The intensity of 1849 8 resonance emission from the cell was determined as a function of the pressure of the various gases. Because the emission was not detectably broadened over the range of gas pressures employed, the peak height of the emission was taken as representative of the intensity. Linear Stern-Volmer plots of (emission intensity) -l us. pressure were reported previously.' In principle, the ratio of the slopes of such lines should represent the ratio of the quenching rate constants. However, the uncertain role of radiation imprisonment required that the technique be tested with quenching systems, the relative cross sections of which were known with good precisioon. Analogous quenching measurements at X 2537 A were performed with H2, He, Ne, "3, Kz,CO, and CH4.* Stern-Volmer treatment of the intensities a t gas pressures from to a t least 50 Torr and normalization of the slopes9 to that for nitrogen gave relative quenching rate constants from which the relative quenching cross sections presented in Table I were calculated.1° The extent of the agreement of these values and those reported in the literature2 establishes the validity of the technique a,t X 2537 8. Extrapolation of the validity to X 1849 A is Table I : Relative Cross Sections for the Quenching of Hg( *PI) Atoms C%S?
Quenoher
H2 He Ne CHI NHa
co
a
This worka
25 0.0 0.0 1.lG 14 23
Estimated error limits &lo%.
The Journal of Physical Chemistry
(Nz = 1.0) Lit.b
31.4 0.0 0.0 0.31 15.4 21.5 'See ref 2. 'See ref 8.
n
He I
I
5
IO
Pv
15 torr
I
I
I
20
25
30
Figure 1. Stern-Volmer plots of quenching data. Each division of ordinate scale represents one unit. Positions of lines with respect to ordinate scale are arbitrary.
based on the reasonable assumption that the 100-times shorter radiative lifetime of Hg('Pl), compared to that of Hg(3P1), does not introduce any complicating features. Stern-Volmer plots of the quenching data at X 1849 8 are shown in Figure 1. I n most cases the points represent averages of replicated experiments. The estimated error limits of the data are of the order of 10%. I n the case of NH3 and CzH4, the direct absorption of X 1849 8 radiation had to be taken into account; for all other gases, absorption a t this wavelength may be neglected. The data for NH3 and C2H4 were first corrected for absorption using published values of extinction coefficients.",l2 The uncertainties in the latter values are reflected in the range of the corrected
*
(8) We thank Mr. V. Madhavabn for carrying out the measurements on CH4. His source of X 2537 A radiation was less intense than that used in the work reported here resulting in a decrease in signal-tonoise ratio. The error in the CH4 is probably greater than the 10% limits estimated for the other data. (9) The Stern-Volmer slope in this experiment is the ratio of the quenching rate constant t o a quantity which, due to imprisonment of radiation, is not the rate constant for fluorescence. This quantity is a constant for fixed geometry, wavelength, and irradiation conditions and disappears in the evaluation of relative quenching rate constants a t both X 2537 A and h 1849 A. (10) Reference 2, p 73.
RELATIVE CROSSSECTIOXS FOR
THE
QUENCHING OF Hg(lPI) ATOMS
quenching dat'a for these gases. Relative quenching cross section values are given in Table 11. Table I1 : Relative Cross Sections for the Quenching of Hg( 'PI) Atoms U21640
Quenoher
MQ
2
4 16 17 18 20 28 28 40 44 44 78 200
.7%7
(Nz = l . O ) a
(Nt = l.O)b
0.2 0.0 0.3
31 0.0 0.3 15 5.2 0.0 21 135 0.0 13 66 75
0.8-1.6O
0.9 0.3 1.0 1.0-2. oc 1.1 0.9 1.3 3.0 2.8
...
a Estimated error limits 3~10%. See ref 2. RaFge reflects the uncertainty in the extinction coefficient at X 1849 A.
Quenching data for Nz, CO, and Ar obtained in the fluorescence cell with 180" geometry gave Stern-Volmer plots with virtually identical slopes, in good agreement with the measurements taken with the standard 90" configuration.
Discussion The results confirm the previous observations that Hg(lP1) is far less selective than Hg(T1) in the transfer
4291
of energyleading to the quenching of resonance fluorescence or phosphorescence. The small dependence of ~ ~ 1 8 on 4 9 the molecular mass but not the chemical nature of the quenching gas stands in stark contrast to the sharp variations of u22637 which have been correlated13 with the electron donating properties of the quencher through the formation of a well-defined collision complex with the electrophilic Hg(3P1) atom. Even the rare gases display this trend although the quenching efficiencies of He and Ne are less than those of other gases of comparable mass. The quenching of Hg('P1) by Ne and Ar implies the conversion of electronic to translational energy since these monatomic species do not have electronic energy levels accessible within the 6.67 eV energy range available from Hg( lP1). The data provide no firm basis for lengthy speculation on the mechanistic details of the quenching process. However, the quenching by Ne and Ar and the apparently simple dependence of u21849 on molecular mass suggests that there is a common, major, quenching mechanism for all the gases studied. With the diatomic and polyatomic gases, other quenching mechanisms can operate simultaneously and, although probably responsible for only a minor portion of the total quenching, may lead to the chemical and scintillation effects observed in Hg(lP1) photosensitization. (11) E. Tannenbaum, E. M. Coffin, and A. J. Harrison, J. C h m . Phys., 21,311 (1953). (12) L.C.Jones, Jr., and L. W. Taylor, Anal. Chem., 27, 228 (1955). (13) H. E. Gunning and 0. P. Strausz, "Advances in Photochemistry," Vol. 1, W. A. Noyes, Jr., G. S. Hammond, and J. H. Pitts, Jr., Ed., Interscience Publishers, New York, N. Y.,1963,p 209.
Volume 73, Number 18 December 1969
