J . Phys. Chem. 1987, 91, 2073-2075 as possible pathways for nucleophilic attack. An important feature of the -NO group (which has no equivalent in -NOz) is the lone pair on the nitrogen. This produces the strongest negative electrostatic potential that is associated with each molecule and should be regarded as a very reactive site (toward electrophiles).
Acknowledgment. We greatly appreciate very helpful dis-
2073
cussions with Dr. Jane S. Murray. We are also most grateful for the support of this work by the US.Army Research Office, and by the Large Caliber Weapon Systems Laboratory, U S . Army ARDEC, Dover, NJ, through the Battelle Scientific Services Program. Registry No. IV, 586-96-9; X, 21354-00-7; XI, 49805-84-7; XII, 659-49-4.
Chemiluminescence from the Ca" (3P) 4- SF, Reaction: Absolute Cross Section, Photon Yields, and Electronic Branching E. Verdasco, V. Slez Rlbanos, F. J. Aoiz, and A. Gonzilez Ureiia* Departamento de Qu;mica Fhica, Facultad de Quimicas, Universidad Complutense, Madrid 28040, Spain (Received: April I , 1986)
A study of the chemiluminescence undef single-collisionconditions of the reaction of the metastable Ca (4s4p 'P") of atomic calcium with SF6 is presented. Chemiluminescence cross sections and photon yields for production of various CaF (A,B) band systems are also reported. The observed electronic branching ratio uA/ugis 4.77, and a comparison with several statistical model calculations is also discussed.
The luminescence in reactions of metastable alkaline-earth atoms with dihalogens1v2and other molecules3-' has been widely studied, and this shows very interesting reaction dynamics since more than one electronic exit channel is involved; therefore the problem of electronic energy disposal has drawn considerable attention.' In addition, molecular beam studies of the alkali atom plus SFsS-12have shown statistical behavior9 as well as the presence of some statistical-dynamical mechanisms involving a Li+-F-SF< intermediate,I0 with the vibrational energy in the LiF bond determined by dynamics and statistical behavior for the other degrees of freedom. Recently we have incorporated a hot oven for metastable atom production in our molecular beam apparatus13 to study (electronic) state-to-state excitation functions. Here we present the product chemiluminescence, obtained at, low collision energy and under single-collision (beam-gas) conditions, of the reaction of the metastable Ca (4s4p 3p0) of atomic calcium with SFs. We shall present chemiluminescence cross sections and photon yields for production of various C a F (A,B) electronic states and discuss the observed electronic branching ratio in light of several (statistical) model calculations. A somewhat higher resolution chemiluminescence spectrum for
+
(1) Menzinger, M. The M X,Reactions: A Case Study in Gas-Phase Chemiluminescence and Chemionization; Fontijn, A., Ed.; Elsevier Science: Amsterdam, 1985. See Also: Menzinger, M. Adv. Chem. Phys. 1980, 42, 1. (2) Kowalski, A.; Menzinger, M. Chem. Phys. Letr. 1981, 78, 461. (3) Telle, H.; Brinkmann, U. Mol. Phys. 1980, 39, 361. (4) Brinkmann, U.; Schmidt, V. H.; Telle, H. Chem. Phys. Lett. 1980, 73,
530.
(5) Irvin, J. A.; Dagdigian, P. J. J . Chem. Phys. 1980, 73, 176. (6) Irvin, J. A.; Dagdigian, P. J. J . Chem. Phys. 1981, 74, 6178. ( 7 ) Dagdigian, P. J. Chem. Phys. Lett. 1978, 55, 239. (8) Behrens, R., Jr.; Herm, R. H.; Sholeem, C . M. J. Chem. Phys. 1976, 65, 4791. (9) Riley, S. J.; Herschbach, D. R. J . Chem. Phys. 1973, 58, 27. (10) Mariella, R. P., Jr.; Herschbach, D. R.; Klemperer, W. J . Chem. Phys. 1973, 58, 3785. (11) Fruend, S . M.; Fisk, G . A.; Herschbach, D. R.; Klemperer, W. J . Chem. Phys. 1971, 54, 2510. (12) Bemnewitz, H. G.;Haerten, R.; Muller, G . Chem. Phys. Lett. 1971, 12, 335. (1 3) Aoiz Moleres, 1981, 59, 61.
F. J.; Herrero, V. J.; Gonzalez Ureiia, A. Chem. Phys.
TABLE I: Experimental Conditions oven temp, K discharge voltage, V discharge current, A
1373
30 1.5
SF6 pressure, mTorr av reactant velocity, c m d TABLE II: Reaction Energies in the Ca System (in eV)"
1.8-7.3 8.8 x 104
+ SF6
-
CaF + SFS
product (CaF) and reactant (Ca) 'S 'Po )PI 'P2
X28+
A*II
1.728 3.608 3.618 3.628 4.438
-0.317 1.563
'D "From ref 22 and 14.
B W -0.612
C2II -2.027
1.268
-0.147
1.573 1.583
1.278 1.288
-0.137 -0.127
2.393
2.098
0.683
+
the Ca (3P) SF6 reaction has been previously reported by Zare and co-~orkers,'~ but only the B-X emission was reported without any consideration to the electronic branching ratio for production of the two A,B band systems, which is one of the main results of the present report.
Experimental and Beam-Gas Characterization The present experiments were carried out in the beam-gas configuration by using part of our molecular beam apparatus described previously." The metastable Ca source follows closely that of Dagdigian's group.5 Essentially it consisted of a (stainless steel) heated effusive oven where ground-state Ca atoms were excited to metastable (3Pand ID) by a low-voltage dc discharge. The excitation was accomplished by floating the crucible a t a negative dc potential with respect to the heater tube potential near the source orifice. The Ca beam passed into a scattering cell of 24-mm total length, through a collimating hole (7-mm diameter), located 80 mm from the source, containing flowing SFs gas, and chemiluminescence was viewed perpendicular to the beam at a distance of 90 mm ~~~
~~
~
~
~
(14) Kiang, T.; Ester, R. C.; Zare, R. N. J . Chem. Phys. 1979, 70, 5925. See also: Kiang, T.; Zare, R. N. J . Am. Chem. SOC.1980, 202, 4024. (15) Jonah, C. D.; Zare, R. N. Chem. Phys. Lett. 1971, 9, 65.
0022-3654/87/2091-2073%01.50/0 0 1987 American Chemical Society
Verdasco et al.
2074 The Journal of Physical Chemistry, Vol. 91, No. 8, 1987
TABLE III: Chemiluminescence Cross Sections and Photon Yields for the Ca ("p) + SFn Reaction "chcmi
C a F band B band, 515-553 nm A band. 564-631 nm
R
A2
0.13 0.62
55 55
L
" F Lo3
6w
c1T)
Wavelength1 nm
Figure 1. (top) Chemiluminescence spectrum for the reaction of Ca ('Po) with SF6 at 4.3 mTorr. Chemiluminescence from the ground-state reaction was not observed. The factor X represents increases in the detector sensitivity. No correction has been made for the spectral response. The peak positions for the C a F (B2Z+ X2Z+) and C a F (A211 XzZ+) progressions are taken from ref 19. (bottom) Chemiluminescence spectrum for the background reaction Ca (IS)+ SF6when the discharge was turned off.
-
photon vield. % 0.23 1.13
turned off. The low-lying electronic states of CaF and available reaction energies in the Ca SF6 reaction are summarized in Table 11. Emissions from the two electronic states, A and B, of CaF, corresponding to transitions B2Z+ X2Z+ and A211 X22+,are observed. No emission from the C-X band (A,, 330.9 nm) was observed under the present experimental conditions. (Note that this spectral range is not shown in Figure 1.) The spectral shape was found to be independent of SF6 pressure over Torr, showing that single-collision the range (0.8-8) X conditions applied for the present experiments. As it has been shown,7 the absolute chemiluminescence cross section is given by
+
O2
%tr
A2
-
from the source. Typical background pressures in the source chamber were 10" Torr and never exceeded 3 X IO" Torr when SF6 was present in the reaction chamber. Typical operating conditions are summarized in Table I. The chemiluminescence was dispersed with a 0.20-m monochromator (Yobin-Ybon) coupled to a RCA C31034 photomultiplier tube connected to an amplifier (Keithley 427) and then to a strip chart recorder. The spectral response of our monochromator, photomultiplier, etc., was determined with calibrated mercury and cadmium lamps. Under the working conditions Ca (ID) may be present, and therefore with the discharge, the densities of Ca (ID) and Ca (3P) were monitored via the ID-% and 3P-1S forbidden transitions at 457.5 and 657.3 nm, respectively. Following the same procedure as reported elsewhere,6v7we have found a ratio of Ca (ID)to C (3P), i.e., n(lD)/n(3P) = 0.33 f 0.10 Results and Discussion
Chemiluminescence Cross Section. First of all to provide a quantitative data base for comparison and photon yield determination, we have measured the absolute cross section for the Ca (3P) SF6 total beam loss quenching. This was accomplished via the usual procedure of monitoring the pressure dependence of the intensity of the metastable decay at 657.3 nm for which suitable leak calibration of the SF, was used to measure its density over a pressure range from 0.3 to 8.5 mTorr. Following a similar procedure as described elsewhere,16 we found a uta of 5 5 A2 for the total attenuation cross section. A previous calibration was carried out by measuring the attenuation cross section with the NzO, obtaining a cross section of 65 A2 which is very close to the reported value of 66.9 A2 from ref 6. Figure 1 shows the (low-resolution) chemiluminescence spectrum together with background spectrum when the discharge was
+
(16) See for example: Yokozeki, A,; Menzinger, M. Chem. Phys. 1976, 14, 427.
-
-
-
where Sa, is the wavelength-integrated signal from CaF in a given spectral region or band. S3+is the Ca (3P1 IS)signal integrated over the spectral bandwidth and corrected for the spectral response of Figure 1. A(3P - ' S ) = (2.9 f 0.2) X lo3 s-] is the atomic transition probability," nSF6is the number density of SF6, and fi = 0.88 X lo5 ems-' is the average relative velocity of the reactant assuming an effusive Maxwellian distribution for the Ca beam. It should be noted that Sa, for the A and B bands was estimated by subtracting the underlying continuum emission from the total chemiluminescence signal. This broad structureless continuum cannot be considered background light from the source since it disappears as the discharge is turned off (see the bottom part of Figure 1). In spite of our low resolution, it could arise from emission of the triatomic CaF2* similarly to the BaClZ*continuum observed by Jonah and C12 beam-gas experiment. Work is in Zarels in their Ba progress to record this continuum emission with a higher resolution as a function of the scattering gas pressure. It will be compared with beam-beam measurements also in progress in our laboratory. Our main emphasis in this present report is on the electronic branching ratio. The fraction of Ca ()P) in the J = 1 level has not been measured, and we have used&, = 0.3 from ref 7 where the distribution of the 3P, states was measured in similar conditions to those of the present experiments. An important question arises when one considers which state, Le., the Ca (ID) or Ca (3P), is responsible for the observed chemiluminescence. To solve this question, we have used the analysis of the pressure dependence of the chemiluminescence cross section as described e l ~ e w h e r e .Essentially ~ for every band and metastable, we can write
+
u~ a ICa€/I&SF6
where ZCaF is the intensity of the head of the band of the C a F emission, Ii that of the atomic line emission, and psF6the SF6 pressure. Figure 2 shows such evolution over the experimental SF6 pressure range of 2-8 mTorr, for the bands assuming the reaction to be due t o Ca (3P) or Ca(lD). The observed constant values for both electronic states over the SF6 pressure imply that the chemiluminescence is predominantly due to the reaction of the Ca (3P) rather than Ca ('D)from which no such constant behavior was found. Table I11 summarizes the obtained cross section together with the respective quantum yields defined as 4CL
=
~OOuCL/~tot
Electronic Branching. It is interesting to compare the observed branching ratio in different electronic states of the C a F with (17) Bauschlichev, C. W., Jr.; Langhoff, S. R.; Jaffe, R. L.; Partige, H. J . Phys. E 1984,17, L427. See also: Fukuda, K.; Veda, K. J . Phys. Chem. 1982, 86, 676.
Chemiluminescence from the Ca* (3P)
I
t
I
-:
I
I
+ SF6 Reaction
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2075
I 1
1
L
%
Q
t-
0.8
82 I+-
x2 '1
o l T i
0.8
+
1 u
Ot
2
L 6 psa. 10'1 torr
8
F w 2. SF, pressure dependence of the chemiluminescent cross sections for the two CaF* bands, assuming the chemiluminescence is due to the reaction of Ca ('F"') (bottom) or C a (ID)(top). TABLE I V Experimental vs. Calculated Electronic Branching Ratio
present exptl result calculated atom-diatom (n = 5/2) collision complex (8 atoms)Ob forming a diatomic plus a nonlinear polyatomic tight (n = 16.5) loose (n = 15.5)
4.77 3.36 61.6 50.0
Referred to in ref 9 and 16. Forms a diatomic plus a nonlinear polyatomic tight or loose transition state.
quantitative model predictions. To this end we have considered the electronic branching ratio r = uA/UBsince it does not depend on any assumption on the cross section for the ground-state formation. Table IV compares the experimental results with several model calculations. Two models were adopted: (a) the R R H O approximation,Is Le., the SF5is considered as an atom, and (b) the full complex estimation, Le., the collision complex (18) See for example: Faist, M. B.; Levine, R. D. Chem. Phys. Lett. 1977, 47, 5 .
formed by the full eight atoms forming a diatomic molecule plus a nonlinear polyatomic f r a g m e r ~ t , ~assuming ?'~ either a tight or loose transition state. It should be noted that the experimental value (4.77) lies closer to the R R H O estimation (3.36) than to the full complex estimation as would correspond to an unimolecular decomposition with all the internal degrees of freedom active, 61.6 (50.0), for a tight (loose) transition state (see Table 111). The classic picture of an electron jump mechanism involving the SF6molecule is commonly associated with the formation of an stable SF6- anion and of an intermediate M+-6F6- molecule-ion that does not dissociate immediately after the electron jump takes place.9 Indeed energy partitioning among products' reaction has been used to fit the K, Rb, and Cs + SF6recoil speed distribution: the CsF vibrational distribution"J2 from Cs SF,, and more recently" the excitation function of Ba, Sr SF6to produce BaF', SrF+ SF5-. However anomalous electronic energy partitioning has been reported in the Mg*, Ca* (3P) C12, Fz reactions2 as an indication of strong dynamical bias as the source of the statistical deviation. Considering that the best n value (of the branching ratio) that reproduces the experimental result is about n N 4.5, one could speculate about some sort of statistical-dynamical mechanism involving a Ca+.-F--SF5- intermediate with the electronic energy in the Ca-F bond determined by dynamics and perhaps statistical behavior for (some o f ) the other degrees of freedom. This type of intermediate was stated to explain the observed nonstatistical LiF vibrational distributionlo combined with the approximately statistical product recoil energy distribution from the Li + SF6 reaction also measured: by the molecular beam method. In any case this mechanistic discussion must be considered rather speculative, and more documented conclusions would require additional information for, for example, the vibrational distribution of the CaF in both electronic channels as well as in the dark one, measured by laser-induced fluorescence. The collision energy dependence of the reported branching will also provide some light into the full dynamics of the reported system." This sort of study will be in progress in our laboratory.
+
+
+
Acknowledgment. We thank V. J. Herrero for his help in part of the experiments and M. Menzinger for his advice. A.G.U. thanks P. J. Dagdigian for his information on the hot oven design. Support from the Comision Asesora of Spain (Grant PB85/007) is gratefully acknowledged. Registry No. Ca, 7440-70-2; SF,, 2551-62-4; CaF, 13827-26-4. Rabinovitch, B. S.; Waage, E . Y. Chem. Reu. 1970, 70, 377. Ross, U.; Meyer, H. J.; Schulze, Th. Chem. Phys. 1984, 84, 359. GonzPlez Urefia, A. Adu. Chem. Phys., in press. Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure; Van Nostrand: New York, 1979; Vol. IV. See also: Herzberg, G. Molecular Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand: New York, 1966. (19) (20) (21) (22)
