The Journal of
Physical Chemistry VOLUME 98, NUMBER 22, JUNE 2,1994
0 Copyright 1994 by the American Chemical Society
LETTERS Multiphoton Ionization Spectroscopy of PClz Radicals: Observation of Two New Rydberg States Jeffrey L. Brumt and Jeffrey W. Hudgens’ Chemical Kinetics and Thermodynamics Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received: March 4, 1994; In Final Form: April I I, 1994”
Two new Rydberg states of the PC12 radical were observed using mass-resolved resonance enhanced multiphoton ionization (REMPI) spectroscopy. -The band systems between 440-480 and 380-395 nm are assigned to b 2A1(4slX 2Bl and E(4p) X 2 B transitions, ~ respectively. These band systems produc_esignal through 2 2(D), and 2 I@) R E M P I processes. Analyses yielded the spectroscopic constants: D 2A1(4s) (uw = 42 760 f 15 cm-l, v’l = 620 f 20 cm-l, and v’2 = 230 f 20 cm-I); E(4p) (vw = 51320 f 10 cm-I, v’l = 600 f 15 cm-I, and u’2 = 240 f 15 cm-l) and 22B1 (v”1 = 525 f 10 cm-l). Previously reported infrared absorptions [ J . Phys. Chem. 1969,73,2774] are reassigned as v”1 = 525 cm-I and v”3 = 452 cm-l. A b initio calculations yielded the optimized geometries, vibrational frequencies, and ionization potentials of PC12(%B1), PC12+(AlBl), and PC12+(B3B1). A b initio G 2 calculations predict the adiabatic ionization potential IP,(PCl2) = 8.5 1 eV.
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Introduction This report, which describes two new electronic states of the transient phosphorus dichloride radical (PC12), is the first of several resonance enhanced multiphoton ionization (REMPI) spectroscopic studies of transient radicals containing group Va atoms. Future reports will describe new electronic states of PF2.I PH,’ PF,’ PCl,2 A s F ~and , ~ AsC1.2 Evidence indicates that group Va radicals have roles during surface etching of semiconductor surfaces,3s4photochemistry,5 and chemical waste disposal. REMPI spectroscopy offers a sensitive and selective optical detection method for each of these group Va radicals. Spectroscopic data pertaining to the PC12 radical are sparse. Investigators have reported infrared6 and ESR spectra’ of ground state PCl2 codeposited in an argon matrix. These studies indicate that ground-state PCl2 has a bent geometry and C2” symmetry. The only published electronic spectrum of PCll exhibits a NRC/NIST Postdoctoral Associate 1992-94. Current address: Environmental Research Laboratories, SmithKline Beecham Pharmaceuticals, King of Prussia, PA. * To whom correspondence should be addressed. Abstract published in Aduance ACS Absrracts, May 15, 1994.
structureless emission between 360 and 800 nm and gives evidence for an A state near 27 000 cm-lS5 Physical properties relevant to REMPI spectroscopy, such as the adiabatic ionization potential of PCl2 and the vibrational frequencies of PC12+, are unknown. We report results of ab initio calculations which aided spectral assignments by estimating these unknown properties.
Experimental Procedures The apparatus is described in detail elsewhere.* Briefly, PCl2 radicals were produced by reacting PH3 with an excess of atomic chlorine within a flow reactor (pressure = 200-400 Pa, flow velocity = 1-3 m/s). Atomic chlorine was produced by passing mixtures composed of 20-3076 Cl2 in helium through a microwave discharge. The radicals effused from the reactor into the ionization region of a time-of-flight mass spectrometer. Radicals were ionized by the focused output (focal length = 150 mm) of an excimer-pumped dye laser (energy = 10-20 mJ/pulse; bandwidth = 0.2 cm-I, fwhm). A computerized data acquisition system collected and averaged the mass-resolved signal.
This article not subject to U S . Copyright. Published 1994 by the American Chemical Society
Letters
5588 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 TABLE 1: Properties of PC12 and PC12+ Obtained from ab Iddo Calculations. r(P-CI), A ~(Cl-P-cl), deg ul(a1) sym str, cm-l 4 a l ) bend, cm-I u3(b2) asym str, cm-l zero-point energy, cm-1 MP2/6-31G* results total energy, hartreeC IP,, eV IP,, eV G2 results total energy, hartreed
IP,, eV IP,, eV
2.045 102.36 496 (525b*c) 20 1 491 (452)c 594
1.948 106.29 585 237 610 716
1.954 122.92
-1 260.078 425 9 8.70 8.47
-1 259.767 324
-1 259.688 567
-1 260.408 922 3 8.72 8.51
-1 260.092 240 0
Optimized geometries are predicted at the MP2/6-31G* theory level. Vibrational frequencies are calculated at the UHF/6-31G* level of theory and then reduced bv 11%. ExDerimental frequencies are listed in parentheses. Result of this gas-phase study. Revised assignment from the Ar matrix study of ref 6. Sek text. 1 hartree = 219 474 cm-I.
Two-Photon Energy (cm-I)
Results and Analysis Calculations. Resonance-enhanced multiphoton excitations of open-shell species generally access Rydberg electronic ~tates.~JO To a good approximation, the wave function of the Rydberg electron is separable from the electronic wave function of the molecular core (i.e., \kpcl2 = * ~ c-\kpc~,+). ~ a Since the bonding within the molecular core of the Rydberg radical resembles that of the cation, we can estimate the geometry and vibrational frequencies of PC12 Rydberg states by performing ab initio calculations on PC12+. Previous studies have obtained useful estimates of the properties of Rydberg radicals using this approach.I1 The ground state of PCl2 has the electronic configuration ...(3b,)2(2a2)2(8b2)2(1 la1)2(4b,)1
% 'B,
46000
45000
I " "
1 .
I
I
" '
'
I
42000 ' ' ' ' I
450
460
'
I . . . .
. . I . . . . I . . . . I . . . .
440 Ground-state PClZ+(% lA1) is formed by removing the 4bl electron. The Rydberg states of PClz associated with the first ionization potential are formed by promotion of the 4bl electron into atomic-like Rydberg orbitals centered on the phosphorus atom. Table 1 shows the results of our calculations on PC12(k2BI), PC12+(%'A1), and PC12+(ii3Bl). Ab initio results were obtained using the Gaussian 92 programs.12J3 Table 1 displays the optimized geometries obtained at the MP2(FU)/6-31G1 level of theory. The geometry of PC12(%2BI) is similar to that found previously using a restricted Hartree-Fock approach.14 There are no previous calculations of PC12+. Table 1 lists the scaled vibrational frequencies of PCl2 and PC12+obtained fromUHF/6-3 lG* calculations. Sincecomputed vibrational frequencies are consistently 9-1 3% higher than the corresponding experimental values,ls Table 1 reports ab initio frequencies which have been reduced by 11%. Experimental frequencies of the PC12 radical are reported in parentheses. Table 1 lists the adiabatic ionization potential (IP.) and the vertical ionization potential (IP,) of PCl2 obtained at the MP2/ 6-31G* and Gaussian-2 ( G 2 ) theory levels.l6 Both values of IPa(PC12) include zero-point energy corrections. We derive IP, using the total energy of PC12+(X1AI) which is frozen at the optimized geometry of PC12(X2Bl). The G2 calculations predict that IPa(PC12) = 8.51 eV and IP,(PC12) = 8.72 eV. These predictions appear reliable. Other studies have found that the G2 method predicts IP,'s to within fO.l eV of their experimental values.1' To check the accuracy of the G 2 procedure with phosphorus radicals, we calculated the G2 IP,'s of PH2 and PF2. We obtained IP,(PH2) = 9.72 eV and IP,(PF2) = 8.84 eV, which agree well with the experimental values, IP,(PH2) = 9.82 eV1* and IP,(PF2) = 8.85 eV.I9 Our calculated geometric parameters for PCl2 displayed in Table 1 are in good accord with recently published calculations based upon density functional theory.20
43000
44000 " "
470
Laser Wavelength (nm) Figure 1. Composite REMPI spectrum of the P35C12radical ( m / z 101)
observed between 440 and 480 nm.
Table 1 also lists the energy and geometry of the lowest electronically excited state of PC12+. MP2(FU)/6-3 lG* calculations pr_edict that PC12+(13B1)resides -2.1 eV higher than the PC12+(X1AI)structure and 10.6 eV above PCl@B1). The error introduced by spin contamination in calculations upon doublet and triplet states is reduced with increasing basis set size. The 6-31G* basis set (as used in our geometry optimizations) is enerally shown to be of sufficient size in this regard.14 The ( 2 ) expectation values -associated with our calculations are 0.768 and 2.02 for the XZBl and 83B1 states respectively. These values indicate that spin contamination is minimal. Experimental Results. The flow reactor effluent produced REMPI spectra carried by m / z 101 (P35Cl2+), 103 (P35C137C1+), and 105 (P37c12+).These spectra are nearly identical, and their intensities are in the expected ratios of -9:6: 1, which reflects the natural abundances of the 3sCl and 37Cl isotopes in PC12. Production of REMPI spectra required the presence of Clz and a microwave discharge indicating that the PCl2 radicals arose from a reaction chain within the flow reactor. Vibrational analyses and all chemical evidence support assignment of the REMPI spectra to the PC12 radical. REMPI spectra were carried only by PC12+,and no evidence for ion fragmentation was observed. This study surveyed t_he spectrum between 300 and 500 nm. An upper state, labeled D, produces the m / z 101 REMPI bands that appzar between 440 and 480 nm (Figure 1). An upper state, labeled E, produces m / z 101 REMPI bands appearing bet_ween 380 and 395 nm (Figure 2). For each REMPI band of the D and B states Table 2 lists Xlaser, the intensity maximum; the two-
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The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5589
Two-Photon Energy (cm-') 52000 1
380
51000 "
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l
3 85
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390
395
Laser Wavelength (nm) Figure 2. REMPI spectrum of the P35C12radical ( m / z 101) observed between 380 and 398 nm.
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TABLE 2 Observed m/z 101 RFMPI Bandg Arising from the DZAl(4s) X2Bl and the E(4p) X2B1 Transitions of the PCll Radical' two-photon freq band A b r (air), nm energy (vac), cm-1 interval! cm-1 assignment B2Al(4s) 42 240 42 760 43 390 43 630
473.3 467.6 460.8 458.3 456.3 454.0 451.8 448.1 442.1
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43 820 44 040 44 260 44 630
g2Bl -520 0 630 870 1060 1280 1500
45 240
1: 000 1: 1;2: (@ 3 b
1; 1; 2;
1870 2480
1; 1:
-530 0 70 500
1: 0: 1;
600 840
1: 1;2;
E(4p) -R2B1 393.7 389.6 389.0 385.8 385.1 383.3
50 790 51 320 51 390 51 820 51 920 52 160
The measurement uncertainty of most bands is f 15 cm-I. Tentative assignment.
TABLE 3 Summary of the Spectroscopic Constants Derived from the REMPI Spectra of PCl2 state T0,O cm-I vl(al) sym str! cm-I vz(a1) bend! cm-I g2B I Q2Al(4s)
E(4p) a
0 42 760 f 15 51 310 f 10
525 f 10 620 f 20 600 f 15
*
230 20 240 f 15
Absolute uncertainty is indicated. l o uncertainty is indicated.
photon energy; the energy relative to the origin band; and the transition assignment. Table 3 lists the derived spectroscopic constanjs. The D state exhibits a prominent, regularly spaced, vibrational progression comprising five bands which terminates at 467.6 nm. Another band at 473.3 nm lies to the red of terminus. The frequency interval between the terminus and the 473.3-nm band is smaller than the intervals along the main progression. This change in vibrational spacing indicates that the 467.6-nm band is the 0; transition. If we assume that the b state is prepared by two-photon resonances, the 473.3-nm band lies 520 cm-1 to the red of the 0; band. This interval is close to our predicted
frequency for the VI' mode of PC12(22B1) (Table 1). Therefore, we assign the 473.3-nm band as the 1: hot band. The prominent vibrational progression extending from the 0; band yields an upper-state vibrational frequency of 620 f 20 cm-I. This frequency lies near the calculated vibrational frequencies of both the vl(al) symmetric stretch and vj(b2) asymmetric stretch modes of PC12+. Because Cb symmetry is conserved in the ground and excited states, the ~3(b2)asymmetric stretch mode will remain inactive. Furthermore, the selection rules governing thevibrational transitions are Au3 = 0, f 2 , f 4 ... for the ~3(b2)asymmetric stretch mode and Aui = 0, f l , f 2 ... for the vl(al) symmetric stretch and v2(al) bend modes.ll Assignment of the progression to the ~3(b2)asymmetric stretch mode would imply the implausible frequency, v'3 = 310 cm-1. Therefore, we assign the progression to originate from the v'l(al) symmetric stretch mode of the upper state. The v'2 bend and v'l symmetric stretch modes also produce 1A2Aand li2Acombination bands which appear -230 cm-1 to the blue of the 1; and 1; bands, respectively. The upper state frequency v'2 = 230 cm-I nearly matches the ab initio frequency of the PC12+ bend, v2 = 237 cm-1. We determine the type of Rydberg orbital using the Rydberg formula: v,(cm-')
= IP,(PC12) - 109737/(n -
(1)
where represents the excitation energy of the 0; band, n is the principle quantum number, and 6 is the quantum defect. Theoretical calculations21 and atomic spectra of phosphorus atoms22 indicate that Rydberg orbitals centered upon phosphorus atoms should exhibit quantum defects of 6(ns) 1.9, 6(np) 1.5, and 6(nd) 0.1. Solving the Rydberg equation for the b state origin (vm= 42 760 cm-I) yields only one reasonable result, n = 4 and 6 = 2.0. This quantum defect indicates that the upper state has an occupied 4s Rydberg orbital. Thus, we designate the upper state as the D2A1(4s)Rydberg state. For thesecalculations we adopt our ab initio result, IP,(PC12) = 8.51 eV. Attributing thespectrum to two-photon absorptions which form Rydberg radicals clarifies the photoabsorption process- that produces the ion signal. Between 435 and 480 nm, PC12(D2AI(4s)) Rydberg radicals may ionize by absorbing two more laser photons. Therefore, the ion signals are generated through a 2 2 REMPI mechanism. The state produces nearly the same pattern of bands as the state. The two-photon energy separating the strong band at 389.6 nm and the weaker band at 393.7 nm is 2hv = 530 cm-I, which supports assignments of the 0; and 1; bands, respectively. The 1; band at 385.1 nm shows that v'1 = 600 cm-I. A band at 385.8 nm is currently unassigned. We are unable to assign the symmetry of this state without further information since in the C2, point group the px, py, and pz orbitals are each a separate represectation. The reasonable solution of the Rydberg equation for the D origin (vm = 51 320 cm-I) is n = 4 and 6 = 1.49. This quantum defect supports the assignment of a 4p Rydberg state. One additional laser photon is required to ionize PC12 radicals from the E(4p) state. Therefore, the ion signals arise from a 2+1 REMPI mechanism.
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a
Discussion
We have found two new electronic states of the transient PC12 radical using resonance-enhanced multiphoton ionization spectroscopy. These spectra appear between the laser wavelengths of 473 and 383 nm and arise through two-photon resonances with Rydberg states that reside between 42 760 and 52 200 cm-1. Most of the vibrational bands originate from the dl symmetric stretch mode of the upper state. The activity observed in the Y'~ mode of PC12 is consistent with the ab initio calculations which predict
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The Journal ofPhysica1 Chemistry, Vol. 98, No. 22, 1994
that theP-Clbondlengthshortensby -0.1 A betweentheground state and the cation-like Rydberg core. In their report of the IR matrix spectrum of the PCl2 radical Andrews and Frederick noted that because the IR selection rules provided no guidance, their assignments of the V I and v3 absorption bands might be inverted. The rigid selection rules which govern the REMPI spectra remove any ambiguity regarding the assignments of the V I and v3 modes. The appearance of hot bands located 525 cm-1 below each origin establishes that v ” ~= 525 cm-1. Since this matches the absorption observed in the IR spectrum, we conclude that the assignments of Andrews and Frederick were inverted. Therefore, the revised assignments for PC12 in an Ar matrix are J’1 = 525 cm-l and ~ ” 3= 462 cm-I. The new vibrational frequency order, v1 > v3, matches the order observed in several other C b triatomic radicals composed of atoms with similar masses, e.g., NF2, CF2, S i c l ~ . * ~
References and Notes (1) Howe, J. D.;Ashfold, M. N. R.; Hudgens, J. W.; Johnson, 111, R. D., manuscript in preparation. (2) Brum, J. L.; Hudgens, J. W., manuscript in preparation. (3) Qin, Q. Z.; Lu, P. H.; Zhuang, Z. J.; Zheng, Q. K. Chem. Phys. Lett. 1992,192, 265. (4) Zin, Z . K.;Li, S.Y.; Li, Y. L.; Yu,M.; Qin, Q. Z. Chinese J. Chem. Phvs. 1991. 4. 249. - -,(5) Bramwell, M. J.; Jaeger, S.E.; Whitehead, J. C. Chem. Phys. Lett. 1992,196,547. (6) Andrews, L.; Frederick, D.L. J . Phys. Chem. 1969,73,2774. (7) Bonazzola, L.; Michaut, J. P.; Roncin, J. J . Chem. Phys. 1981,75, 4829. (8) Johnson 111, R. D.;Tsai, B. P.; Hudgens, J. W. J . Chem. Phys. 1989, 91,4558. (9) Hudgens, J. W. Advances in Multi-photon Processes andSpectraFcopy; Lin, S.H.,Ed.; World Scientific: Singapore, 1988. - 7
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Letters (10) Ashfold, M. N. R.; Clement, S.G.; Howe, J. D.;Western, C. M. J . Chem. SOC.,Faraday Trans. 1993,89,1 1 53. (11) See,for example: (a) Hudgens, J. W.; Johnson 111, R. D.;Tsai, B. P.;Kafafi,S.J.Am.Chem.Soc.1990,112,5763. (b)Dearden,D.V.;Hudgens, J. W.; Johnson 111,R. D.;Tsai, B. P.; Kafafi, S.J . Phys. Chem. 1992,96,585. (c) Brum, J. L.; Johnson 111, R. D.; Hudgens, J. W. J. Chem. Phys. 1993,98, 3732. (d) Hudgens, J. W.; Johnson 111, R. D.;Tsai, B. P. J. Chem. Phys. 1993,98, 1925. (12) GAUSSIAN 92, Revision A; Frisch, M. J.; Trucks, G. W.; HeadGordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Rohb, M. A.; Replogle, E. S.;Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; Martin, R. L.; Fox, D.J.; Defrees, D.J.; Baker, J.;Stewart, J. J. P.;Pople, J. A. Gaussian Inc.,Pittsburgh PA, 1992. (13) Certain commercial materials and equipment are identified in this paper in order to adequately describe the experimental p r d u r e . In no case does such an identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment is necessarily the best available for the purpose. (14) Hinchliffe, A.; Bounds, D.G. J. Mol. Srruct. 1979,54, 231. (1 5) Hehre, W.; Radom, L.; Schleyer,P. R.; Pople, J. A. Ab initioMolecular Orbital Theory; Wiley-Interscience: New York, 1986. (16) Curtiss, L. A.;Raghavachari, K.; Trucks, G. W.; Pople, J. A. J . Chem. Phys. 1991,94,7221. (17) Curtiss, L. A.; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J . Chem. Phys. 1991, 94,7221. (1 8) Berkowitz, J.; Curtiss, L.;Gibson, S.;Greene, J.; Hillhouse, G.; Pople, J. J. Chem. Phys. 1986,84,375. (19) Berkowitz, J.; Greene, J.; Foropoulos, J., Jr.; Neskovic, 0. M. J. Chem. Phys. 1984,81,6166. (20) Gutsev, G. L. Chem. Phys. 1994,179,325. (21) Thecdosiou, C. E.;Inokuti, M.; Manson, S.T. At. Data Nucl. Data Tables 1986,35, 473. (22) Moore, C.E.At. Energy Levels, Natl. Stand. Ref. Daia Ser. 1949, 461. (23) Jacox, M. E.VibrationalandEIectronicEnergyLeoelsofPolyatomic Transient Molecules, J . Phys. Chem. Ref. Data Monograph No. 3, 1994.
