J. Phys. Chem. 1991,95,4291-4296
4291
proposad here, a thermal excitation or activation energy of 2500 cm-' is required for indole to populate vibrational levels in the 'Lb state which couple effectively with the 'La state to produce a vibronic state that is subject to efftcient nonradiativedecay. In the case of 3-methylindole, it is possible that the methyl substituent provides lower energy vibrational modes in the 'Lb state which lead to nonradiative decay.
bronic mixing of the 'La and 'Lb excited singlet states of these molecules. Vibronic coupling involving higher vibrational levels also promotes radiationless deactivation of the lowest excited singlet state in fluid polar solvents. As a result of vibronic mixing, the 'La and 'Lb states of indole and 3-methylindole lose their identity in polar solvents, but the fluorescent emission is from a state of principally 'Lb character.
Conclusions The substantial Stokes shift observed with the fluorescence emission spectrum of indoles in polar solvents is attributed to a pseudo-Jahn-Teller effect resulting from thermally induced, vi-
Acknowledgment. We gratefully acknowledge the financial support provided by the Wool Research and Development Fund Grant MISO2T from the Australian Wool Corporation. Registry No. Indole, 120-72-9; 3-methylindole, 83-34-1.
New Rydberg States of Aluminum Monofluoride Observed by Resonance-Enhanced Multiphoton Ionization Spectroscopy David V. Dearden: Russell D. Johnson III,* and Jeffrey W. Hudgens* Chemical Kinetics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received: September 13, 1990; In Final Form: January 2, 1991)
AIF was generated in the gas phase with high-temperature reduction of AIF, by AI. AIF was detected and characterized with 1+2,2+1,2+2, 3+1, and 3+2 resonance-enhanced multiphoton ionization spectroscopy. Thirteen new Rydberg states that lie between 70000 and 77 000 cm-I were identified for the first time. These new states in combination with previously known states were organized into six Rydberg series. Least-squares fitting of the Rydberg series to the Rydberg equation yields the adiabatic ionization potential of AIF, IP, = 9.729 & 0.001 eV. Vibrational intervals of the new Rydberg states are about 25% greater than those of AIF(X'Z+), with most lying between 930 and 980 cm-I.
Introduction
1a22d3$4$1d5$6a22r47a2
Aluminum monofluoride exhibits an abundant electronic spectrum. Previous spectroscopic studies' have characterized eight singlet and seven triplet excited electronic states. Even so, questions regarding the electronic structure of AlF remain. For example, although previous studies have proposed that six of the excited singlet states have Rydberg character, spectra of the higher Rydberg states that would confirm these assignments have not been reported. In addition, the assignments of II and A states in the triplet manifold are somewhat uncertain, prompting detailed theoretical studies of the excited states of A1F.24 Although AIF is a transient species under ambient conditions, gas-phase AIF is easily produced at moderate temperatures by heating the trifluoride in the presence of metallic aluminum. Our interest in AIF stems in part from our ongoing effort to use resonance-enhanced multiphoton ionization (REMPI) to detect and characterize transient species important in semiconductor fabrication. Since large amounts of spectroscopic information are already available for AlF excited states, this molecule serves as a useful test case for how well our techniques can be applied to metal halides. REMPI has proven to be a very effective tool for investigating Rydberg states of group IV free radicals,5+ suggesting it should also be appropriate for transient group 111 species. Therefore, a further goal of this work was to identify AIF Rydberg series and to use this new information to verify the earlier tentative Rydberg assignments. Our understanding of AIF is greatly enhanced by a number of theoretical studies."JO The ground state of AIF is X ' P . According to ab initio calculations, at internuclear separations close to the equilibrium separation the orbital occupancy is best described asll NIST/NRC Postdoctoral Associate, 1989-90. Current address: Department of Chemistry, University of Texas at Arlington, Arlington, TX 76019.
The 7a orbital has primarily AI 3s character? This agrees with our intuitive expectation that bonding in AIF should be ionic, with the A1 3p electron largely transferred to F. At larger separations, the orbital occupancy changes and is better described aslo 1a22a23$4$ 1r45a26a22?r47a'8a1 The Sa orbital exhibits primarily A1 3p character,2 and the molecule dissociates to ground state atoms, A1(2P) + F(2P). The AIF+ cation has also been studied by using ab initio methods,'O with the finding that at equilibrium separation the 7a orbital has mixed A1 3s3p character, while at longer range it is primarily A1 3s. The ground state, X2Z+, and the first excited 2 state, B 2 P , undergo an avoided crossing at r = 3.8-4.0 bohrs. Thus, the ground state of the cation dissociates diabatically to Al+(,P) + F(2P), but due to the avoided crossing the adiabatic dissociation limit is to Al+('S) + F(2P). (1) Previous spectroscopic work on AlF is summarized in Barrow, R. F.; Kopp, I.; Malmberg, C. fhys. Sci. 1974, IO, 86-102. (2) So, S. P.; Richards, W. G. J. Phys. B Ar. Mol. fhys. 1974, 7,
1973-1980. (3) Hint, D. M.J. Mol. Spectrosc. 1987, 121, 189-198. (4) Langhoff, S.R.; Bauschlicher, C. W., Jr.; Taylor, P. R. J. Chem. fhys. 1988,88, 5715-5725. (5) Johnson, R. D., 111; Fang, E.; Hudgens, J. W. J . Phys. Chem. 1988.
92, 3880. (6) Hudgens, J. W.; Dulcey, C. S.;Long, C. R.; Bogan, D. J. J . Chem. Phys. 1981,87,4546-4558. (7) Johnson, R. D., 111; Hudgens, J. W. J. fhys. Chem. 1989, 93, 6268-6 270. (8) Johnson, R. D., 111; Tsai. B. P.; Hudgens. J. W. J . Chem. fhys. 1989. 91, 3340-3358. ( 9 ) Johnson, R. D., 111; Tsai, B. P.;Hudgens, J. W . J . Chem. fhys. 1988, 89, 4558. (IO) Klein, R.; Rosmus, P. Theor. Chim. Acra 1984, 66, 21-29. (1 1) Dyke, J. M.;Kirby, C.; Morris, A.; Gravenor, B. W. J.; Klein, R.; Rosmus, P. Chem. fhys. 1984,88,289-298.
This article not subject to U.S.Copyright. Published 1991 by the American Chemical Society
4292 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
Dearden et al.
The lowest energy Rydberg states of AlF possess a cation core with the same configuration as the ground state cation (X2Z+). They are generated from the neutral by promotion of a 7a electron into Al-centered Rydberg orbitals.' Transitions from the ground state of AIF to the various Rydberg states roughly correspond to excitation of an A1 3s electron into a Rydberg orbital centered on Al. This is in agreement with the intuitive expectation that the positive charge in the AlF+ cation should reside on the less electronegative A1 atom.
I
Experimental The procedures and apparatus used in our REMPI studies of transient species have been described in detail.9 Early efforts to generate gas-phase AlF by the reaction of atomic F with Al(CH3)3 resulted in some spectra, but AlF production by that method was erratic. Much more stable and reproducible results were achieved by using a reductive heating method which has previously been described." AlF was generated by placing a mixture of metallic aluminum (2N8 mesh, Strem Chemicals) and AlF, (98.5%, Strem Chemicals) powders in a resistively heated tantalum foil oven located approximately 2 cm from the ionization region of a time-of-flight mass analyzer. The oven was slowly heated until a peak appeared in the mass spectrum at m / z 46. This occurred at temperatures below the point at which a visible glow was observed from the oven, estimated at less than 800 OC. More precise temperature measurements were not made, since the samples were heated inhomogeneously. lonization of the neutral species effusing from the oven was accomplished by using the output of a pulsed dye laser, focused with a 150." focal length lens. The ions were mass resolved, and the intensities of the ion masses of interest were monitored and averaged with a gated integrator, as a function of laser wavelength. A XeCl excimer laser was used to pump the dye laser in most experiments (dye laser output: 10-35 mJ/pulse, 25-11s duration, 0.2-cm-' bandwidth). The spectra shown are composites obtained with the laser dyes (Exciton Chemical Co.) PTP (330-350 nm), DMQ (345-375 nm), QUI (371-405 nm), DPS (397-415 nm), Stilbene 420 (411-431 nm), Coumarin 440 (420-464 nm), Coumarin 460 (440-484 nm), Coumarin 480 (476-498 nm), and Coumarin 510 (490-540 nm). Spectra to the blue side of 330 nm were obtained by using the frequency-doubled output of a Nd:YAG pumped dye laser operating with the dye DCM (doubled output; 15-25 mJ/pulse, 10-ns duration, 0.2-cm-' bandwidth). No compensation has been made for variations in laser energy occurring from dye to dye or across the tuning range of a given dye. Results and AM~YWS REMPI detection is extremely sensitive for AIR with the oven heated strong signals were obtained. Figures 1 and 2 show the m / z 46 REMPI spectra of AIF between 305 and 540 nm. The same optical spectra were also carried very weakly at m / z 27 (AI+). During these experiments ion signals corresponding to A1F2+or AlF3+were not observed. All spectral features produced vibrational frequencies characteristic of AlF excited and ground electronic states. Thus, we assign the carrier of these spectra to AlF. In the figures, the major bands are marked with electronic and vibrational state assignments. Table I provides a comprehensive list of band maxima. For each band maximum, Table I lists the REMPI mechanism that produced the m / z 46 signal, the energy of the upper electronic state,I3 the upper electronic state assign(12) Certain commercial materials and equipment are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply rccommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment identified is necessarily the best available for the purpose. (13) The energy uncertainty associated with each band maximum is a function of laser wavelength. The uncertainty is also multiplied by the numkr of photons used to prepare the resonant upper state. In general, the energy uncertainty associated with each band maximum is about 1 2 0 cm-I.
330
335
340
345
350
355
370
375
380
385
390
395
400
410
415
420
425
430
435
440
445
450
455
450
455
460
465
470
415
400
485
490
495
360
365
405
370
410
375
415
I
I
I
3
495
500
505
510
515
520
525
530
535
540
Laser Wavelength (nm) Figure 1. REMPI spectrum of AIF ( m / z 46) observed between 334 and 540 nm.
Two Photon Energy (cm-') YOpOO
310
I
62poO
320
6 4 y
,
,
330
Laser Wavelength (nm) Figure 2. REMPI 2+1 spectrum of AIF ( m / z 46) observed between 305 and 333 nm.
ment, and the vibrational quanta of the upper (u') and lower (u") electronic states. As noted in Table I, the spectra originate from 1+2, 2+ 1, 2+2, 3+ 1, and 3+2 REMPI mechanisms. The total
New Rydberg States of Aluminum Monofluoride
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4293
4294 The Journal of Physical Chemistry, Vol. 95, No. 1 1 , 1991 TABLE II: AIF k t r o n i c States Detected ia "him Work
cm-I this prior work study vm
state# X'Z+ a% A'II
BIZ+ C1Zt(4su)
D'A(3db) E'II(3dr) Flll(4pr) G1Z+(3do) H'Zt(4pu)
sn(ar)
J'II(5pr) K'Z'(4do) LIZt(Spa)
typeb V V V V R
R R R R R R
0 27256 43960 54298 57787 61280 63769 65889 66427 67408 70567 71433 71652 72111 72526 73577 74009 74060 14364 75101 75422 75471 76247
0 21254 43950 54282 57756 f 63750 65872 66406 67397
AGl,;,cm-l this prior AGllF, work study cm-l 822 805 844 926 887 918 954 926 961 963 95 1 1017 932 962 940 946 977 934
820 792 852 928 889 912 946 915 944
793d 792
e 770 793
e 800 777
e 780 788 806 772 783 818 792 790 784 762
R R R M'Z'(6su) R N'II(5dr) R 0111(6pr) R P'E+(Sde) R Q1Z+(6po) R [R111(6dr)]# R C e S'II(7pr) R 964 786 T'Z+(6do) R 940 790 U1Z+(7du) R 977 e #Character of Rydberg orbital in parentheses. b V = valence state. R = Rydberg state. CFromref 16. dFrom ref 14. uNot observed. fThe D-X transition has not previously been observed. #Tentativeassignment. photon order, 3-5 photons, is assigned by the requirement that the sum of all laser photons must exceed the adiabatic ionization energy of AIF, 9.73 eV." Table I1 summarizes the electronic states observed in the REMPI spectra. For each electronic state, Table I1 lists the vw energy; AG1/i, the (u' = 1)-(0' = 0 ) vibrational interval in the excited state; and AGI~",the (0'' = l)-(v" = 0) Vibrational interval in the ground state. Microwave spectroscopy has accurately established that AG1/F = 793 cm-' in the ground state of A1F.I4 The hot bands assigned during the analysis of most REMPI band systems also yield values for AG, F. Thus, an entry in Table I1 that lists AG1/," = 793 cm-I is also evidence that the electronic origin and photon order assignments for the resonant state are correct. In the following sections, we provide brief descriptions of each REMPI band system. For clarity we have divided the following data presentations into two sections, one that describes REMPI spectra of previously characterized electronic states and a second that describes the assignment of new electronic states. Identification of REMPI Bands Originating from Previously Known States. Previous optical studies of AIF have identified eight excited singlet states and seven triplet states. In the REMPI spectra, transitions from the X'Z" state to all of the previously characterized singlet states are observed. The only REMPI bands from a triplet state were assigned to the all& state. For comparison with the present REMPI results, Table I1 lists the vw and.AGlp' reported from earlier optical studies. Within the experimental uncertainty, these spectroscopic constants agree. a'n, XIZ+Bands. The transition a3n, XIZ+was seen as a 1+2 REMPI process with a band maximum a t 366.79 nm (lhv = 27 256 cm-I). Although this transition is formally forbidden, previous workers observed its spectrum in emission from a flame formed by the reaction of gaseous A1 with SF6,NF3, or Fz.Is Our transition energy agrees closely with a value of vw = 27 254 cm-I from the emission study, corroborating prior mea+
-
(14) (a) Lide, D. R. J . Chem. fhys. 1965,42, 1013. (b) Meki, A. G.; Lovas. F. J. J . Mol. Specrrosc. 1982. 95. 90. (c) Wyse, F. C.;Gordy, W.; Peanon, E. F. J . Chem. fhys. 1970, 52, 3887. (d) Hoeft, J.; Lovas, F. J.; Tiemann, E.; Torring, T. Z. Narurjorsch. 1970, 2 5 4 1029. (IS) Rosenwaks, S.;Steele, R. E.;Broida, H. P. Chem. fhys. tea. 1976, 38, 121-124.
Dearden et al. surementlJs and calculation4 of the singlet-triplet separation energy. The one-photon nature of these REMPI bands is confirmed by the observation of the vibrational u' = 0 u" = 1 hot band and the spin-orbit splitting. An assignment of this band system to one-photon resonances gives AGl/," = 792 cm-', in agreement with the expected AG,,," = 793 cm-I. The vibronic origin band is split into five rotational branches, as expected for a 311 IZ+ transition when the 311 state conforms to Hund's case a. Similar splitting is also observed in the u' = 0 u'' = 1 hot band and in the u' = 1 or' = 0 band. The band structure of the rotational branches is consistent with the previously reported spin-orbit coupling constant A = 47 cm-I.l6 A'nXIZ+Bands. The A ' n state was prepared through a 2+2 REMPI mechanism. The origin is observed at 454.84 nm (2hv = 43 960 an-'),which agrees closely with the literature value, vw = 43 950 Only one excited vibrational level of the A state is clearly evident, as the signal is overlapped by those from threephoton resonances with a number of higher lying states. The hot band is obscured by overlap with the strong E state threephoton resonant bands. BIZ+ X'Z+ Bands. The origin of this 2+1 REMPI transition appears at 368.23 nm (2hv = 54298 cm-') which agrees with the literature value, vw = 54 282 cm-I.l6 The presence of the hot band and the u' = 2 , l - u" = 0 bands (Table I) confirms the electronic state assignment. The weak peak a t 535.36 nm is also assigned to the B (2-0) transition from a 3+2 resonance. CIZ+-XIZ+Two- and Three-Photon Resonances. The C-X transition is observed through 2+1, 3+1, and 3+2 REMPI transitions. The origin band appears as a two-photon resonant transition at 346.13 nm and as a three-photon resonance at 519.01 nm. The corresponding two- and three-photon energies, 2hv = 57 765 cm-I and 3hv = 57 787 cm-', are in good agreement with the previously reported vw = 57 756 ~m-'.'**'~A number of u' u" = 0 bands appear as both two- and three-photon resonant transitions. The spectra display extensive hot-band activity. Sequence bands are particularly distinct in the 3+1 REMPI spectrum but are also seen in the 2+1 REMPI spectrum. D'A XIZ+Bands. Under one-photon selection rules, IA-lZ+ transitions are forbidden because the change in total orbital angular momentum, AA, is *2 and one-photon transitions can accommodate only angular momentum changes of AA = 0, f 1 . Consequently, prior one-photon absorption studies of AlF have not observed the D-X band system. To characterize the DIP state, previous studies have analyzed the intense one-photon DIA A 1 n emission spectrum.' D-X transitions are allowed when they are excited by simultaneous absorption of two or three photons. During this study, D X bands appeared as a series of strong 2+1 REMPI transitions (Figure 2). The intense origin observed at 326.28 nm (2hv = 61 280 cm-I) agrees closely with literature term values which predict vw = 61 278 cm-I.I6 In agreement with prediction,16 the D(l 0) and D(2 0) bands lie 887 and 1791 cm-' to the blue side of the origin. The expected hot band should lie under the C(3 0) band and is not identified in the spectral assignments. Curiously, no ion signal attributable to three-photon resonances with the DIA state was detected in this study. E'n-X'Z' Two- and Three-PhotonResonances. The E 1 nstate produces intense 2+ 1 and very intense 3+ 1 REMPI spectra. The origin is observed at 31 3.62 nm (2hv = 63 754 cm-I) and at 470.31 nm (3hv = 63 769 cm-I), in good agreement with the previously reported voo = 63 750 cm-1.18919The photon order of each band system is verified by the presence of hot-band structure, which reproduces the ground-state vibrational spacings.
-
-
-
+
-
-
-
-
-
-
+
(16) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure, IV. Constants of Diaromic Molecules; Van Nostrand: New York,
1979. (17) Rowlinson, H. C.; Barrow, R. F. froc. fhys. Soc. A 1953,66,437. (18) Rowlinson, H. C.; Barrow, R. F. froc. fhys. Soc. A 1953,66,771. (19) (a) Barrow, R. F.; Rowlinson, H. C. froc. R. Soc. A, 1954,224. 143. (b) Naud€, S.M.; Hugo, T. J. Con. J . fhys. 1955, 33, 573. (c) Naudt, S. M.; Hugo, T.J. Can. J . fhys. 1957, 35, 64.
The Journal of Physical Chemistry, Vol. 95. No. 11, 1991 4295
-
New Rydberg States of Aluminum Monofluoride
F'n X'Z+Bands. The F-X bands appear as a series of 3+1 REMPI transitions. The origin lies at 415.18 nm (3hu = 65 889 cm-I) in excellent agreement with the literature value, uw = 65 872 ~ m - ' . ' ~ . The ' ~ F(O 1) hot band and the F(l 0), F(2 0), and F(3 0) transitions are also assigned. G'Z+ XIZ+Bands. The G-X bands appear in the REMPI spectrum as a series of 3+1 REMPI transitions. The origin is assigned to the 451.50-nmband (3hv = 66427 cm-I) in good agreement with the literature value, vw = 66406 cm-1.18919 The REMPI spectrum also displays bands from the u' = 0, 1,2, 3 , 4 u" = 0 transitions at energies that agree with the earlier s t u d i e ~ . ' ~The J ~ G(O 1) hot band, if present, is obscured by 1) hot band. the relatively strong E(3 H'Z+ X'Z+ Bands. The H-X bands appear in the REMPI spectrum as 3+1 REMPI transitions. The origin appears at 444.92 nm (3hu = 67 408 cm-I) as a shoulder on the G( 1 0) band within experimental error of the previously reported origin at uW = 67 397 c ~ - ~ . I * J ~The presence of the H-X band system is confirmed by the assignment of the clearly resolved H(O 1) hot band and H( 1 0) and H(2 0) vibronic bands (Table I). Identification of New Electronic States. Prior to this work the H state was the highest lying known state of AlF. Previous studies have tentatively classified the singlet electronic states that lie above the B state as Rydberg in nature,' correlating with the ground state of A l p . Dyke et al." have suggested that the C, D, E, F, G, and H states comprise the lowest energy members of six different Rydberg series. Between 380 and 440 nm, the REMPI spectrum exhibits many relatively weak, previously unobserved bands. The energy range of three photons (68 200-79000 cm-I) is the region where we expect to observe the higher Rydberg states associated with the first ionization potential of 9.73 eV. Table I lists the analyses of these new REMPI bands. In order of their priority, these assignments were found and verified by using the following criteria: (1) Since the oven produces vibrationally hot AlF, each assigned band system should display one (or more) vibrational hot band($, which confirms the origin and photon order assignments by yielding AGIl2" 793 cm-I. (2) Since the vibrational spacings in Rydberg states are expected to resemble the cationic state to which they correlate, assignments should produce electronic-state vibrational intervals of AGIl2'= 920-1000 cm-I. The expected range for AGI i is based upon ab initio calculations of AlF+(X2Z+), which predict we 960 cm-I, and upon the photoelectron spectrum of AlF, which reported o, = 1040 i 40 cm-I for AIFC(X E+)." (3) When appropriate, the new Rydberg origin should fit into a Rydberg series. Table I1 summarizes the properties of the 13 new electronic states identified with the assignment criteria. Except for the R'n(6d7r) and U1Z+(7du) states, the new states meet all assignment criteria. Although the R state fits into a Rydberg series, its assignment remains tentative due to the absence of hot-band and vibrational structure. The U state origin assignment is rendered uncertain by congestion that obscures the U(0 1) hot band. The following short descriptions of the new band systems emphasize the electronic origin assignments from vibrational analyses. The discussion of the Rydberg symmetry assignments is presented in later sections. I'II(4dr) xlZ+ Bunds. We have assigned the I-X band system to a 3 1 REMPI mechanism. The relatively intense I state origin lies at 424.96 nm (3hu = 70567 cm-I). Our assignment of the I(0 1) hot band at 429.75 nm yields AGI/Z= 788 cm-', which properly reproduces the ground-state vibrational interval. The three member progression (u' = 0, 1, 2) along the upper state (Table I) yields the spectroscopic constants we = 1005 cm-' and o s e = 21 cm-'. J'II(5pr) XIZ+ Bands. We have assigned the J-X bands to originate from a 3+1 REMPI mechanism. The origin lies at 419.86 nm (3hu = 71 433 cm-I). The J(0 1) hot band that appears as a shoulder on the I state origin yields the satisfactory value, AGl F = 806 cm-I. The three member progression (0' = 0, 1, 2, 3) aiong the upper state (Table I) yields the spectroscopic constants o, = 955 cm-l and o s e = 2.0 cm-l.
-- - -
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- +
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-
-
-
-
+
+ -
-
-
K1Z+(4da) XIZ+Bunds. We have assigned the K-X bands to originate from a 3+1 REMPI mechanism. The K state origin lies at 418.57 nm (3hu = 71 652 cm-I). The origin band coincides with the expected position of the 1(2 1) hot band but is more intense relative to the I( 1 0) band than is reasonable for the hot band. This overall assignment scheme receives additional support from the K(O 1) hot band assignment at 423.13 nm, which yields a satisfactory AGI/F = 772 cm-' and by the K( 1 0) band assignment at 413.06 nm which yields a reasonable AG1/i = 1017 cm-I. L 2+(5pa) X'Z+ Bands. The sharp peak at 415.91 nm (3hu = 72 1 1 1 cm-I) is assigned to the origin of the L-X system. The origin assignment is supported by the L(0 1) hot-band assignment, which gives AG1/F = 783 cm-I, and the L( 1 0) assignment, which gives A G 1 / i = 932 cm-I. M1Z+(6su) XI2 Bunds. The origin of the M X three-photon transition at 413.53 nm (3hv = 72 526 cm-I) lies in a region of spectral congestion. The M(O 1) hot band at 418.25 nm appears as a shoulder on the K state origin. In view of the distortion introduced by congestion, these assignments yield an acceptable AGllf" = 818 cm-I. The M(l 0) band at 408.1 1 nm yields AGII2 = 962 cm-'. N'n(5d7r) X'Z+ Bands. The origin of the N X three-photon transition appears as a distinct peak at 407.62 nm (3hv = 73 577 cm-I). The N(O 1) hot band contributes to the congestion around the M state origin and gives the expected AG1/( = 792 cm-l. The N(l 0) band yields AG,p' = 940 cm- . OlII(6pr) X'Z+ Bands. The origin of the 0 X three-photon transition lies at 405.24 nm (3hv = 74009 cm-I). 1) hot band at In support of this origin assignment the O(0 409.62 nm gives AGIIF = 790 cm-I, which closely matches the expected value. The O( 1 0) band yields AG!/i = 946 cm-I, which is consistent with a Rydberg state vibrational frequency. P'2+(5da) XIZ+ Bunds. The P-X origin band lies at 404.96 nm (3hu = 74060 cm-I), adjacent to the 0 X origin. In support of this origin assignment, the P(0 1) hot band and the P( 1 0) assignments give the satisfactory values of A G 1 / F = 784 cm-' and A G 1 / i = 977 cm-I, respectively. Q1Z+(6pa) XIZ+Bunds. The distinct peak at 403.31 nm (3hv = 74 364 cm-I) is assigned as the three-photon origin of the Q X transition. The Q(0 1) hot band appears only 25 cm-' to the blue side of the N state origin band, which could distort its apparent position and account for the smaller than desired ground-state vibrational interval, AG1/F = 762 cm-'. The Q(l 0) band yields A G I I i = 934 cm-I. R'II(bd7r) XIZ+Bund. The peak at 399.35 nm (3hu = 75 101 cm-I) is assigned to the R-X three-photon origin. No hot or vibrational bands were observed for this state, so the assignment rests entirely on the agreement of this peak with the prediction of the Rydberg equation that a 6d7r Rydberg state should be found at 75 124 cm-'. Therefore, the assignment of this band origin is tentative. S111(7p7r) XIZ+Bands. The S X origin lies at 397.77 nm (3hu = 75 398 cm-I). In support of the assignment of this band system to three-photon transitions, the very weak S(0 1) hot band yields a satisfactory A G l / F = 786 cm-'. The distinct S( 1 0) band at 392.75 nm yields A G l l i = 964 cm-I. T1Z+(6da) XIZ+ Bunds. The T X origin lies at 397.39 nm (3hu = 75 472 cm-I). The T(O 1) hot band yields a satisfactory AGI 211 = 790 cm-I. The T(l 0) band which appears as a shoulder on the S(1 0) band yields AGlp' = 940 cm-I. U1Z+(7da) XIZ+Bands. The band assigned as a three-photon resonance with the U X origin appears as distinct peak at 393.35 nm (3hu = 76247 cm-I). No hot band was distinguished, presumably because it is obscured by the S X and T X origins. The U( 1 0) band at 388.37 nm yields AGI/i = 977 cm-I. Rydberg Series of AIF. Table I1 lists 19 Rydberg state origins. Table I11 shows the organization of these Rydberg origins into six different Rydberg series. Table 111 also shows the results of a least-squares fit that uses the Rydberg formula, uw(cm-') = IP,
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-
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Dearden et al.
4296 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
TABLE 111: Fit of P r o w Rydbem Series to Rydberg E4urti01P Rydberg
state C'Z+
M H'Z+
L
Q F'n
J 0
S G'E+ K P T U Eln I
orbital type 4so 6so 4PU 5PU 6PU 4P* 5P7 6P* 7P* 3do 4d a 5do 6do 7do 3d* 4dr 5d7 6d* 3d6
8, Quantum
defect
1.697
* 0.002
0.849 f 0.004
1.047
* 0.003
-0.017 f 0.004
0.269
0.003
Y
~
obsd 57 787 72 526 67408 72111 74 364 65889 71 433 74 009 75 422 66427 71 652 74 060 75471 76 247 63 769 70 576 73 577 75 101 61 280
cm-' ,
calcd 57 184 12 545 67418 71 103 74 336 65 890 71 450 73 999 75 315 66413 71 670 74111 75 440 76 243 63 762 70 590 73 570 75 131 b
-
~al(0bsd)
um(calcd), cm-I 3 -19 -10 8 28 -1
-17
IO 47 14 -18 -5 1 31 4 7 -14 7 -30
N R 0.473 D'A "IP, = 78472 11 cm-l (9.729 0.001 eV). bThis state was not used as part of the least-squares fit.
- 109737/(n -
to calculate the quantum defect, 6, of each series and the adiabatic ionization potential, IP,. As shown in Table 111, the fit of the Rydberg origins is very precise. The largest difference between the observed and calculated energies of each Rydberg origin is less than 5% of a vibrational interval. The quality of this fit increases confidence in the spectroscopic assignments. The quality of this fit increases confidence in the spectroscopic assignments. Misassignment by as little as one vibrational quantum would produce a much poorer fit than is obtained. A comparison of the quantum defects found from the leastsquares fit with quantum defects of atomic aluminum enables Rydberg orbital assignments. Using an unrelaxed HermanSkillman potential, Manso# has calculated theoretical quantum defects of alumhumcentered Rydberg orbitals, which predict that 6 = 1.5 for s states, 6 = 1.0 for p states, and 6 = 0.01 for d states. As shown in Table 111, these predicted values support our assignments of the Rydberg states to form one s, two p, and three d Rydberg series. For further comparison, the compendium by Moore2' lists the experimentally observed energy levels of atomic aluminum, which show that 6 1.8 for s states and 6 1.3 for p states. As is seen in Table 111, in AIF the quantum defects of the ns and np Rydberg series are somewhat smaller than the atomic values. In atomic aluminum the quantum defects of ns and np series are nearly constant along the Rydberg series.21 In contrast, the quantum defect of the nd 2D Rydberg series in atomic aluminum increases from 6 = 0.37 to 6 = 0.95 as the principal quantum number increases from n = 3 to n = 11.21 However, in AlF the
-
-
(20) Manson, S.T.Phys. Reo. 1969, 182, 97-103. (21) Moore, C. E. Atomic Energy Levels. Narl. Stand. Ref.Data Ser. (US.Nail. Bur. Stand.) 1971, 35.
IZ+(ndu) and 'lI(nd7r) Rydberg series change only slightly with principal quantum number. Because of the irregularity along the nd Rydberg series in atomic aluminum, the invariance of nd quantum defects in AlF along each nd Rydberg series (Table 111) was not expected.
Discussion The results of this study show that REMPI is a very sensitive probe for AlF. The higher Rydberg states along a series absorb more weakly than the first series member. Previous optical studies characterized only the lowest energy Rydberg states. In this study, the discovery of 13 Rydberg states of higher principal quantum number illustrates the great sensitivity of REMPI detection for AlF. These results provide confidence that REMPI spectroscopy is a useful tool for the study of transient group 111 metal halides. Table 111 summarizes the IP,, the quantum defects of Rydberg series, and the properties of the least-squares fit. The value IP, = 78472 f 11 cm-' (9.729 f 0.001 eV) is in excellent agreement with IP, = 9.73 f 0.01 eV from photoelectron spectroscopyII and IP = 9.70 f 0.50 eV from electron impact appearance potent i a l ~ . ~ ~The J ' IP, derived from the fit of the Rydberg series offers a more precise value. We find that our present Rydberg state assignments, which are derived from the quantum defects of Rydberg series, agree completely with the assignments by Dyke et al.," who assumed that the C, D, E, F, G, and H states each comprise the lowest lying member of a Rydberg series. Table I1 shows that the vibrational frequency of AlF in each Rydberg state is usually 25% higher than in AlF(X'Z+). This increased vibrational frequency would seem to indicate that the bond energy of AlF+, which has essentially the same bonding properties as the Rydberg states, is stronger than AlF(X'Z+). However, measurements show that AlF+ (00= 72 kcal mol-') is more weakly bound than AlF (00= 159 kcal mol-'). Ab initio calculation^^*^' also predict a weaker bond in Alp. The vibrational frequencies and bond energies can be reconciled in light of the covalent-ionic avoided crossing in the cation. The large AlF+ vibrational frequencies reflect the diabatic potential energy surface, which dissociates to Al+(3P)+ F(2P). This limit is 4.64 eV above the adiabatic Al+(IS) + F(2P) limit:' so the overall depth of the diabatic well is about 179 kcal mol-l, 12% greater than the adiabatic dissociation of the A1F neutral. Thus, AlF is a case in which vibrational frequency and bond energy, 00, do not correlate. Avoided crossings complicate the bond energy-vibrational frequency relationship. The vibrational interval, ACIl2, does indicate the relative width of the vibrational potential energy well near the minimum. The larger AG1/2/ of each cationlike Rydberg state indicates that the potential well of AlF+ is narrower than in AlF. This narrower potential well correlates with the ab initio calculations," which find that the bond len th in AlF+ (re = 1.601 A) is shorter than in AlF (re = 1.663 ). In brief, compared with the case of AlF(X'Z+), the vibrational potential energy surface of A l p is shallower, narrower, and displaced toward a smaller equilibrium distance.
1
Registry No. AIF, 13595-82-9. (22) (a) Ehlert, T. C.; Blue, G . D.; Green, J. W.; Margrave, J. L. J . Chem. Phys. 1964,41,2250-2255. (b) Porter, R. F. J . Chem. Phys. 1%0,33,951. (23) Murad, E.; Hildenbrand, D. L.; Main, R. P. J . G e m . Phys. 1966, 45. 263-269.
