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Observation of the ν1+nν2 combination band in the C̃′ 1A1′ Rydberg state of NH3
Volume 145, number 3
CHEMICAL PHYSICS LE’TTERS
1 April 1988
OBSERVATION OF THE Y,+w2 COMBINATION BAND IN THE i? ‘A; RVDBERG STATE OF NH, Paul J. MILLER, Steven D. COLSON and William A. CHUPKA Sterling Chemistry Laboratory,
Yale University, New Haven, CT 06511, USA
Received 14 December 1987; in final form 11January 1988
A re-investigation of NH3 in the vicinityof the fi stateoriginby 2 + 1multiphotonionizationphotoelectron spectroscopy(MPI PES) has revealed the presence of a previously unreported combination band of the c’ state which involves the Y, symmetric stretch and the Y, umbrella mode. The symmetric stretch frequency ofthe c’ state and of the ground state ion have been measured. What was once thought to be an anomalous 2 + 1 MPI PES of the NH, d state origin is now re-assigned to the I?’ 1‘23 level.
1. Introduction In general, multiphoton ionization photoelectron spectroscopy (MPI PES) is a powerful technique in assigning vibrational levels of relatively unperturbed molecular Rydbergs (recent examples include O2 [ 1] and C2H2 [ 2 ] ). For an n + 1 multiphoton ionization process (where n is the number of photons to the Rydberg resonance), there is a large Franck-Condon overlap in the Au=0 one-photon ionizing transition between the unperturbed Rydberg and the ion, while those for Auf 0 transitions are much smaller. This is a result of the nearly identical geometries of the Rydberg state and the ion state to which the Rydberg is converging. In cases in which a single vibrational level of the ion does not dominate the photoelectron spectrum, the MPI PES technique has been used to probe a variety of phenomena, such as perturbations of Rydbergs by adjacent states [ 3,4], autoionization of states resonant with the ionizing photon [ 5-71, and state-to-state relaxation dynamics [ 81 along with accompanying photodecomposition [ 9 1. Recently, 2+ 1 MPI PES has shown that the u2(a; ) umbrella mode fundamental and overtone levels of the NH, c’ ‘A; and B ‘E” Rydberg states conserve vibrational quanta to a large degree ( > 70%) upon ionization [ lo]. This allows for the efficient production of vibrationally state-selected NH,+ ions and has found an important application in the study of ion-molecule reaction dynamics
[ 11,12 1. In contrast, what was thought to be the 2 t 1 MPI PES of the NH3 B ‘E” 2’ Rydberg state has shown a predominant single vibrational peak not assignable to a strong Au= 0 ionizing transition of the b state itself [ 13 1. If indeed the b 2Olevel was being populated, the photoelectron spectrum indicated an apparent Au= 6 transition (i.e. formation of NH: 2 2’j) occurring almost exclusively. The appearance of this photoelectron peak could not be adequately explained in terms of previously known phenomena that result in non-Franck-Condon behavior. We have re-investigated the MPI PES of the NH3 0 IE” state and have been able to attribute this apparently anomalous photoelectron spectrum to a previously unreported c’ v, + nv2 combination band.
2. Experimental Photoelectron spectra were obtained using a version of the 211electron spectrometer first developed by Kruit and Read [ 141. Photoelectrons were detected by two multichannel plates at the end of a 50 cm flight tube. The resulting signal was collected and averaged by a transient digitizer (Tektronix 76 12D) interfaced to a lab minicomputer (Digital LSI 11/23). In the 2 + 1 MPI studies, the frequency-doubled output of an Nd: YAG-pumped visible dye laser (Quanta DCR-2A, PDL 1, WEX- 1; dyes R6G, Kiton Red, R640) was focused through a 200 mm focal
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length lens into the ionization region. The laser power was kept low to minimize Coulomb broadening of the photoelectron signal. Partial rotational cooling ( T,,<50 K) was obtained by expanding 5% NH3 seeded in Ar through a 26 pm hole at the tip of a cw nozzle. Backing pressure was kept at half an atmosphere and was limited by the pumping speed of the spectrometer. Ionization by the laser radiation occurred about 2 mm downstream from the nozzle tip. A shorter focal length lens (focal length = 75 mm) and an effusive nozzle (with no subsequent rotational cooling) were used in obtaining the 3 + 1 MPI PES of the b 2Olevel. Laser radiation was provided by a XeCl-excimer-pumped UV dye laser (Questek 2240, Lambda Physik FL2002) using the dye coumarin 440. The 2+ 1 MPI wavelength scans were performed either in the photoelectron spectrometer or in a conventional time-of-flight mass spectrometer. In the mass spectrometer, more extensive rotational cooling could be achieved than in the electron spectrometer by using a pulsed nozzle (Lasertechnics LPV) operated at 2 atm backing pressure. The advantage of using the photoelectron spectrometer was that overlapping rotational lines from the B state could be “removed” from the wavelength spectrum by only monitoring the photoelectron peak that corresponded to ionization of the et state.
3. Results and discussion The one-color 2 + 1 MPI PES of NH3 has been obtained at the two-photon energy corresponding to the b 2Ostate (fig. 1a). This is compared to the MPI PES of the c’ 2’j state (fig. lb). It is evident that no significant production of ground state NH: 2’ is occurring via what was previously thought to be the fi 2Olevel. To examine possible effects in the thirdphoton ionization step, a two-color scheme was subsequently used to probe the same intermediate level. Two photons (1~286.98 nm) populated this level and a third, longer-wavelength photon (Nd: YAG third harmonic, A=355 nm) served to ionize. The resulting two-color MPI PES was still lacking the presence of a photoelectron peak corresponding to the formation of NH: 2O,and, aside from the photoelectron energy shift resulting from ionization with 184
1 April 1988
1c2o
,
l-.
I
I
(4 1026
011Jj.G~. -2.0 2.4
2.8
- 3.2*
’
Electron KE (eV) Fig. 1. (a) The 2+ 1 MPI photoelectron spectrum of NH3 ob tained at 1~286.98 nm. This is near the b state origin at the twophoton level, but a photoelectron peak correlating to the l”20level of the ion is not observed. The major peak has been assigned to a Au= 0 ionizing transition from the c’ 1‘23 level (see text). (b) The 2+1 MPI photoelectron spectrum of the e’ 1o26 state (1~288.28 nm) showing a strong Au=0 ionization step. Weaker side bands correspond to the 1’2’and 1’2’levelsof the ion.
a longer-wavelength photon, the spectrum was essentially unchanged from that of the one-color MPI PES. Because of the disparate energies of the ionizing photons in the one- and two-color cases, it is unlikely that the observed one-color MPI PES is due to an accidental resonance with an autoionizing state at the third-photon level. The one-color 2 + 1 MPI wavelength scan of NH, in the energy region of the fi 2’and cf 26 resonances is shown in fig. 2. Rotational cooling has greatly simplified the structure of these bands. The c’ 26 level is a parallel band (AK=O) and its MPI spectrum shows an intense Q transition, while the nearby R and S lines are much weaker. These rotational features are characteristic of all the vibrational levels of the c’ state [ lo]. In contrast, the NH3 B state bands, which are perpendicular transitions (hK= & 1)) display a more complicated profile for even vibrational levels alternating with simpler structure for odd vibrational levels [ 10,151. (The alternation of rota-
Volume 145, number 3
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CHEMICAL PHYSICS LETTERS
28j.O
287.5
288.0
288.5
Wavelength (nm) Fig. 2. The 2 + 1 MPI wavelength scan of NH3 ( T,,z 20 K) in the region of the ?, 1’2’and c’ lo26 bands. The spectrum weas obtained by monitoring the production of NH: in a mass spec= trometer as the laser wavelength was scanned.
tional structure for even and odd levels is a result of having ortho-para nuclear spin forms of NH3. ) The two-photon transition to the fi 2Ostate is expected to be a perpendicular band with a rotational profile similar to an even vibrational level of the B state since both have the same electronic symmetry (E” ). The band in fig. 2 near the energy corresponding to the fi 2’ state, however, shows the simpler structure of a parallel transition and therefore cannot be assigned to the b state. The weak structure at the indicated position of the b state origin could conceivably be due to this state, but the l”20 level of the ion was not observed in the photoelectron spectrum. Wavelength scans to the red of this region reveal three additional bands at intervals corresponding to the vZ umbrella mode frequency. Each band has the same simple rotational structure typical of a parallel transition at low rotational temperature. The lowest member of this progression is located at 67088 cm-‘, which is ~33200 cm-’ above the origin of the (?’ state. It should be noted that the lowest progression member could not be unequivocably assigned by wavelength scans performed in the mass spectrometer due to severe rotational overlap with the B 2’state, even when rotationally cooled, but was readily apparent in wavelength scans performed in the photoelectron spectrometer. We do not believe these bands belong
1 April 1988
to a v2 overtone progression of a new electronic state since no such states having parallel vibronic structure (other than the c’ state) are possible in this energy region. In addition, the observed photoelectron spectrum obtained by 2 + 1 MPI of the band at 67088 cm-’ is not consistent with a 2’ level (fig. 3 ). Due to the close proximity of the c’ origin, and based on rotational profiles, vibrational spacings, and 2+ 1 MPI photoelectron spectra, we are assigning this progression to vX+nv2 combination bands of the e’ state. For planar Rydberg states of NH3 (D3,, symmetry), in addition to the v2(a;) umbrella mode, there are the v, (a; ) symmetric stretch and two pairs of degenerate e’ vibrations. In a two-photon absorption to a vibronic level of the cf state, the only mode that will result in a parallel transition in combination with the vt mode is that of the vl symmetric stretch, thus we can assign these bands to a v1+nv, series. Table 1 gives the measured band positions for n=O to 3. The difference between the lowest observed band and the c’ origin [ 16 ] gives a fundamental frequency of v,(cI NH,)=322322 cm-‘. From the observed photoelectron energies in the 2+ 1 MPI PES, and relying on the conservation of vibrational quanta in the one-photon ionization of these levels, we also obtain v, (2 NH: ) = 3 150 + 100
~...:...:...:_--_.;...:.._;...;...~ 0
516
1032
1548
2064
2580
Flight Time (nsec) Fig. 3. The 2+ 1 MPI photoelectron spectrum of the c’ 1’2Olevel (k298.1 I nm) plotted linear in time. The vibrational assignments given in the figure are those of the ion. The peaks corresponding to the 1o27and 1“2*ion levels are due to the overlapping B 1028state.
185
Table 1 The NH3 c’ v, t nvs combination band Level
E (cm-r)
Av (cm-‘)
1°ZO 1’2” 1’2’ 1’22 1223
63866 ” 61089 67914 68788 69707
3223 825 874 919
a) Fromref
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CHEMICAL PHYSICS LETTERS
Volume 145, number 3
[16].
cm-* for the symmetric stretch of the ground state ion. This is somewhat higher than the value of Y(ji NH: ) w 2700 cm-’ measured in the He1 photoelectron spectrum [ 17 1. This, however, was based on an extrapolation from higher 1’2” combination bands since the 1’2Olevel could not be seen due to overlap with the much stronger 2” overtone progression. We note that Y,= 3337 cm- ’in ground state neutral NH3 [ 181 and the ejected electron is nonbonding. We have also observed the same combination band progression in the 2 + 1 MPI spectrum of ND3 (table 2). From the energy difference of the lowest band position and the ND3 c’ origin 1141, a value of u,(CND,)=2324+2 cm-’ is obtained for the ND1 c’ state. The ratio of v,(~‘NH~)/v,(C’ND,) is 1.39, in reasonably good agreement with the fi isotope effect predicted by the product rule for a harmonic vibration [ 19 1.
Although we are reassigning the peak previously attributed to the NH3 b 2’ state in the 2 + 1 MPI PES [ 131 to the c’ 1‘23 combination band, we are not implying that the strong D state progression in the WV absorption spectrum is mis-assigned. The c’ state is one-photon forbidden and is not observed in the WV spectrum. (The vl totally symmetric stretch would not, of course, make it vibronically allowed.) Apparently, the b state is extremely weak in the 2 + 1 MPI spectrum. One possible cause for this situation is that the ionization cross section of the b state is very low. However, the two-color 2+ 1 MPI PES would indicate it is very low over a substantial energy range, which seems unlikely. Also, the b state becomes quite pronounced once again in the 3+ 1 MPI spectrum [ 201. The 3 + 1 MPI PES of the fi 2’ level (fig. 4) does indeed show a dominant single peak that corresponds to the formation of the vibrationless ground state ion. A second and far more likely possibility is that the b state two-photon transition probability is extremely small relative to that of the c’ state. A far from compelling rationalization can be given for this low probability in terms of united atom selection rules but a reliable explanation will probably require a more valid theoretical treatment.
Table 2 The ND, c’ v, + nv, combination band ‘) Level 102s I220
E (cm-r) 63956 b’ 66280
1’2’ 1’22 1’2’
(66910) 67566 68310
1’24 1’25
69056 69813
Av (cm-‘) ,,,,c L.?‘, (630) (656) 744 -I*19, 757
a) Values given in parentheses are tentative since wavelength scans in either the photoelectron spectrometer or the mass spectrometer were unable to unambiguously locate the ND3 C’ 1’2’band due to an NDsH impurity in the former and overlapping fi state lines in the latter. b)Fromref. [16].
186
Electron KE (eV) Fig. 4. The 3 + 1 MPI photoelectron spectrum of the b 1’2’level (1~430.03 nm). Production of NH: a 1‘23occurs via the overlapping e’ 1’2’ state at the three-photon level. The two peaks marked with asterisks arc unassigned and may be due to ionization of photofragments of NHs.
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4. Conclusion By employing both time-of-flight mass spectroscopy and photoelectron spectroscopy coupled with the rotational cooling of a supersonic expansion, we have been able to re-assign the 2 i- 1 MPI PES of what was formerly thought to be the NH3 b state origin. The new assignment attributes the spectrum to the c’ 1‘23combination band. Four members of the NH3 c’ 122”progression were observed, with the position of the NH3 cf 1’2’ band giving a symmetric stretch frequencyofV,(C’NH,)=3223+2cm-‘.Thesame band progression was also observed in the c’ state ofND, (v~(cl NDs)=2324k2 cm-‘). In addition, from the measured photoelectron energies arising from 2 + 1 MPI of the NH3 e’ 1’2* level, we have obtained the symmetric stretch frequency of the ground state ion (~,(%NH:)=3150flOO cm-‘). The 3+ 1 MPI PES of the correct i?j state origin displayed a single dominant photoelectron peak that could be correlated to production of NH,+ 2 2’. The appearance of a nearly exclusive 2)=0 ionizing transition via 3+ 1 MPI indicates that the fi state is a “good” Rydberg and its absence in the 2 + 1 MPI PES is due to reasons other than a strong mixing with nearby states at the intermediate resonance level.
Acknowledgement The authors wish to thank Dr. William E. Conaway and Dr. Richard N. Zare for making available their 2+ 1 MPI spectrum of NDs. This work was supported by the National Science Foundation (Grant No. CHE-8318419).
I April 1988
References [ I ] A. Sur, C.V. Ramana, W.A. Chupka and S.D. Colson, J. Chem. Phys. 84 (1986) 69. [2]M.N.R. Ashfold, B. Tutcher, B. Yang, Z.K. Jin and S.L. Anderscn, J. Chem. Phys. 87 (1987) 5105. [ 31 M.G. While, M. Seaver, W.A. Chupka and SD. Colson, Phys. Rev. Letters 49 (1982) 28. [4] J.B. Pallix, P. Chen, W.A. Chupka and S.D. Colson, J. Chem. Phys. 84 (1986) 5208. [S] S.T. Pratt, P.M. Dehmer and J.L. Dehmer, J. Chem. Phys. 85 (1986) 3379. [6] W.A. Chupka, J. Chem. Phys. 87 (1987) 1488. [7] A.P. Hickman, Phys. Rev. Letters 59 (1987) 1553. [ 81 J.B. Pallix andS.D. Colson, Chem. Phys. Letters 119 (1985) 38. [9]S.R. Long, J.T. Meek, P.J. Harrington and J.P. Reilly, J. Chem. Phys. 78 (1983) 3341. [ 10] W.E. Conaway, R.J.S. Morrisonand R.N. Zare, Chem. Phys. Letters 1I3 ( 1985) 429. [ I 1 ] R.J.S. Morrison, W.E. Conaway and R.N. Zare, Chem. Phys. Letters 113 (1985) 435. [ 121 R.J.S. Morrison, W.E. Conaway, T. Ebata and R.N. Zare, J. Chem. Phys. 84 (1986) 5527. [ 131J.B. Pallix and S.D. Colson, J. Phys. Chem. 90 ( 1986) 1499. [14]P.KruitandF.H.Read,J.Phys.E16(1983)313. [ 15lB.D. Kay and A.K. Grimley, Chem. Phys. Letters 127 (1986) 303. [ 161M.N.R. Ashfold, CL. Bennett, R.N. Dixon, P. Fielden, H. Rieley and R.J. Stickland, J. Mol. Spectry. 117 ( 1986) 2 16. [ 171J.W. Rabalais, L. Karlsson, L.O. Werme, T. Bergmark and K. Siegbahn, J. Chem. Phys. 58 (1973) 3370. [ 181G. Herzberg, Electronic spectra and electronic structure of polyatomic molecules (Van Nostrand, Princeton, 1966). [ 191 G. HenbeG Infrared and Raman spectra (Van Nostrand, Princeton, 1945). [20] J.H. Glownia, S.J. Riley, S.D. Colson and G.C. Nieman, J. Chem. Phys. 73 (1980) 4296. [21] M.N.R. Ashfold, R.N. Dixon and R.J. Stickland, Chem. Phys. 88 (1984) 463. [ 221 G.C. Nieman and S.D. Colson, J. Chem. Phys. 7 1 ( 1979) 571.
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CHEMICAL PHYSICS LE’TTERS
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OBSERVATION OF THE Y,+w2 COMBINATION BAND IN THE i? ‘A; RVDBERG STATE OF NH, Paul J. MILLER, Steven D. COLSON and William A. CHUPKA Sterling Chemistry Laboratory,
Yale University, New Haven, CT 06511, USA
Received 14 December 1987; in final form 11January 1988
A re-investigation of NH3 in the vicinityof the fi stateoriginby 2 + 1multiphotonionizationphotoelectron spectroscopy(MPI PES) has revealed the presence of a previously unreported combination band of the c’ state which involves the Y, symmetric stretch and the Y, umbrella mode. The symmetric stretch frequency ofthe c’ state and of the ground state ion have been measured. What was once thought to be an anomalous 2 + 1 MPI PES of the NH, d state origin is now re-assigned to the I?’ 1‘23 level.
1. Introduction In general, multiphoton ionization photoelectron spectroscopy (MPI PES) is a powerful technique in assigning vibrational levels of relatively unperturbed molecular Rydbergs (recent examples include O2 [ 1] and C2H2 [ 2 ] ). For an n + 1 multiphoton ionization process (where n is the number of photons to the Rydberg resonance), there is a large Franck-Condon overlap in the Au=0 one-photon ionizing transition between the unperturbed Rydberg and the ion, while those for Auf 0 transitions are much smaller. This is a result of the nearly identical geometries of the Rydberg state and the ion state to which the Rydberg is converging. In cases in which a single vibrational level of the ion does not dominate the photoelectron spectrum, the MPI PES technique has been used to probe a variety of phenomena, such as perturbations of Rydbergs by adjacent states [ 3,4], autoionization of states resonant with the ionizing photon [ 5-71, and state-to-state relaxation dynamics [ 81 along with accompanying photodecomposition [ 9 1. Recently, 2+ 1 MPI PES has shown that the u2(a; ) umbrella mode fundamental and overtone levels of the NH, c’ ‘A; and B ‘E” Rydberg states conserve vibrational quanta to a large degree ( > 70%) upon ionization [ lo]. This allows for the efficient production of vibrationally state-selected NH,+ ions and has found an important application in the study of ion-molecule reaction dynamics
[ 11,12 1. In contrast, what was thought to be the 2 t 1 MPI PES of the NH3 B ‘E” 2’ Rydberg state has shown a predominant single vibrational peak not assignable to a strong Au= 0 ionizing transition of the b state itself [ 13 1. If indeed the b 2Olevel was being populated, the photoelectron spectrum indicated an apparent Au= 6 transition (i.e. formation of NH: 2 2’j) occurring almost exclusively. The appearance of this photoelectron peak could not be adequately explained in terms of previously known phenomena that result in non-Franck-Condon behavior. We have re-investigated the MPI PES of the NH3 0 IE” state and have been able to attribute this apparently anomalous photoelectron spectrum to a previously unreported c’ v, + nv2 combination band.
2. Experimental Photoelectron spectra were obtained using a version of the 211electron spectrometer first developed by Kruit and Read [ 141. Photoelectrons were detected by two multichannel plates at the end of a 50 cm flight tube. The resulting signal was collected and averaged by a transient digitizer (Tektronix 76 12D) interfaced to a lab minicomputer (Digital LSI 11/23). In the 2 + 1 MPI studies, the frequency-doubled output of an Nd: YAG-pumped visible dye laser (Quanta DCR-2A, PDL 1, WEX- 1; dyes R6G, Kiton Red, R640) was focused through a 200 mm focal
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
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length lens into the ionization region. The laser power was kept low to minimize Coulomb broadening of the photoelectron signal. Partial rotational cooling ( T,,<50 K) was obtained by expanding 5% NH3 seeded in Ar through a 26 pm hole at the tip of a cw nozzle. Backing pressure was kept at half an atmosphere and was limited by the pumping speed of the spectrometer. Ionization by the laser radiation occurred about 2 mm downstream from the nozzle tip. A shorter focal length lens (focal length = 75 mm) and an effusive nozzle (with no subsequent rotational cooling) were used in obtaining the 3 + 1 MPI PES of the b 2Olevel. Laser radiation was provided by a XeCl-excimer-pumped UV dye laser (Questek 2240, Lambda Physik FL2002) using the dye coumarin 440. The 2+ 1 MPI wavelength scans were performed either in the photoelectron spectrometer or in a conventional time-of-flight mass spectrometer. In the mass spectrometer, more extensive rotational cooling could be achieved than in the electron spectrometer by using a pulsed nozzle (Lasertechnics LPV) operated at 2 atm backing pressure. The advantage of using the photoelectron spectrometer was that overlapping rotational lines from the B state could be “removed” from the wavelength spectrum by only monitoring the photoelectron peak that corresponded to ionization of the et state.
3. Results and discussion The one-color 2 + 1 MPI PES of NH3 has been obtained at the two-photon energy corresponding to the b 2Ostate (fig. 1a). This is compared to the MPI PES of the c’ 2’j state (fig. lb). It is evident that no significant production of ground state NH: 2’ is occurring via what was previously thought to be the fi 2Olevel. To examine possible effects in the thirdphoton ionization step, a two-color scheme was subsequently used to probe the same intermediate level. Two photons (1~286.98 nm) populated this level and a third, longer-wavelength photon (Nd: YAG third harmonic, A=355 nm) served to ionize. The resulting two-color MPI PES was still lacking the presence of a photoelectron peak corresponding to the formation of NH: 2O,and, aside from the photoelectron energy shift resulting from ionization with 184
1 April 1988
1c2o
,
l-.
I
I
(4 1026
011Jj.G~. -2.0 2.4
2.8
- 3.2*
’
Electron KE (eV) Fig. 1. (a) The 2+ 1 MPI photoelectron spectrum of NH3 ob tained at 1~286.98 nm. This is near the b state origin at the twophoton level, but a photoelectron peak correlating to the l”20level of the ion is not observed. The major peak has been assigned to a Au= 0 ionizing transition from the c’ 1‘23 level (see text). (b) The 2+1 MPI photoelectron spectrum of the e’ 1o26 state (1~288.28 nm) showing a strong Au=0 ionization step. Weaker side bands correspond to the 1’2’and 1’2’levelsof the ion.
a longer-wavelength photon, the spectrum was essentially unchanged from that of the one-color MPI PES. Because of the disparate energies of the ionizing photons in the one- and two-color cases, it is unlikely that the observed one-color MPI PES is due to an accidental resonance with an autoionizing state at the third-photon level. The one-color 2 + 1 MPI wavelength scan of NH, in the energy region of the fi 2’and cf 26 resonances is shown in fig. 2. Rotational cooling has greatly simplified the structure of these bands. The c’ 26 level is a parallel band (AK=O) and its MPI spectrum shows an intense Q transition, while the nearby R and S lines are much weaker. These rotational features are characteristic of all the vibrational levels of the c’ state [ lo]. In contrast, the NH3 B state bands, which are perpendicular transitions (hK= & 1)) display a more complicated profile for even vibrational levels alternating with simpler structure for odd vibrational levels [ 10,151. (The alternation of rota-
Volume 145, number 3
28‘6.5
CHEMICAL PHYSICS LETTERS
28j.O
287.5
288.0
288.5
Wavelength (nm) Fig. 2. The 2 + 1 MPI wavelength scan of NH3 ( T,,z 20 K) in the region of the ?, 1’2’and c’ lo26 bands. The spectrum weas obtained by monitoring the production of NH: in a mass spec= trometer as the laser wavelength was scanned.
tional structure for even and odd levels is a result of having ortho-para nuclear spin forms of NH3. ) The two-photon transition to the fi 2Ostate is expected to be a perpendicular band with a rotational profile similar to an even vibrational level of the B state since both have the same electronic symmetry (E” ). The band in fig. 2 near the energy corresponding to the fi 2’ state, however, shows the simpler structure of a parallel transition and therefore cannot be assigned to the b state. The weak structure at the indicated position of the b state origin could conceivably be due to this state, but the l”20 level of the ion was not observed in the photoelectron spectrum. Wavelength scans to the red of this region reveal three additional bands at intervals corresponding to the vZ umbrella mode frequency. Each band has the same simple rotational structure typical of a parallel transition at low rotational temperature. The lowest member of this progression is located at 67088 cm-‘, which is ~33200 cm-’ above the origin of the (?’ state. It should be noted that the lowest progression member could not be unequivocably assigned by wavelength scans performed in the mass spectrometer due to severe rotational overlap with the B 2’state, even when rotationally cooled, but was readily apparent in wavelength scans performed in the photoelectron spectrometer. We do not believe these bands belong
1 April 1988
to a v2 overtone progression of a new electronic state since no such states having parallel vibronic structure (other than the c’ state) are possible in this energy region. In addition, the observed photoelectron spectrum obtained by 2 + 1 MPI of the band at 67088 cm-’ is not consistent with a 2’ level (fig. 3 ). Due to the close proximity of the c’ origin, and based on rotational profiles, vibrational spacings, and 2+ 1 MPI photoelectron spectra, we are assigning this progression to vX+nv2 combination bands of the e’ state. For planar Rydberg states of NH3 (D3,, symmetry), in addition to the v2(a;) umbrella mode, there are the v, (a; ) symmetric stretch and two pairs of degenerate e’ vibrations. In a two-photon absorption to a vibronic level of the cf state, the only mode that will result in a parallel transition in combination with the vt mode is that of the vl symmetric stretch, thus we can assign these bands to a v1+nv, series. Table 1 gives the measured band positions for n=O to 3. The difference between the lowest observed band and the c’ origin [ 16 ] gives a fundamental frequency of v,(cI NH,)=322322 cm-‘. From the observed photoelectron energies in the 2+ 1 MPI PES, and relying on the conservation of vibrational quanta in the one-photon ionization of these levels, we also obtain v, (2 NH: ) = 3 150 + 100
~...:...:...:_--_.;...:.._;...;...~ 0
516
1032
1548
2064
2580
Flight Time (nsec) Fig. 3. The 2+ 1 MPI photoelectron spectrum of the c’ 1’2Olevel (k298.1 I nm) plotted linear in time. The vibrational assignments given in the figure are those of the ion. The peaks corresponding to the 1o27and 1“2*ion levels are due to the overlapping B 1028state.
185
Table 1 The NH3 c’ v, t nvs combination band Level
E (cm-r)
Av (cm-‘)
1°ZO 1’2” 1’2’ 1’22 1223
63866 ” 61089 67914 68788 69707
3223 825 874 919
a) Fromref
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CHEMICAL PHYSICS LETTERS
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[16].
cm-* for the symmetric stretch of the ground state ion. This is somewhat higher than the value of Y(ji NH: ) w 2700 cm-’ measured in the He1 photoelectron spectrum [ 17 1. This, however, was based on an extrapolation from higher 1’2” combination bands since the 1’2Olevel could not be seen due to overlap with the much stronger 2” overtone progression. We note that Y,= 3337 cm- ’in ground state neutral NH3 [ 181 and the ejected electron is nonbonding. We have also observed the same combination band progression in the 2 + 1 MPI spectrum of ND3 (table 2). From the energy difference of the lowest band position and the ND3 c’ origin 1141, a value of u,(CND,)=2324+2 cm-’ is obtained for the ND1 c’ state. The ratio of v,(~‘NH~)/v,(C’ND,) is 1.39, in reasonably good agreement with the fi isotope effect predicted by the product rule for a harmonic vibration [ 19 1.
Although we are reassigning the peak previously attributed to the NH3 b 2’ state in the 2 + 1 MPI PES [ 131 to the c’ 1‘23 combination band, we are not implying that the strong D state progression in the WV absorption spectrum is mis-assigned. The c’ state is one-photon forbidden and is not observed in the WV spectrum. (The vl totally symmetric stretch would not, of course, make it vibronically allowed.) Apparently, the b state is extremely weak in the 2 + 1 MPI spectrum. One possible cause for this situation is that the ionization cross section of the b state is very low. However, the two-color 2+ 1 MPI PES would indicate it is very low over a substantial energy range, which seems unlikely. Also, the b state becomes quite pronounced once again in the 3+ 1 MPI spectrum [ 201. The 3 + 1 MPI PES of the fi 2’ level (fig. 4) does indeed show a dominant single peak that corresponds to the formation of the vibrationless ground state ion. A second and far more likely possibility is that the b state two-photon transition probability is extremely small relative to that of the c’ state. A far from compelling rationalization can be given for this low probability in terms of united atom selection rules but a reliable explanation will probably require a more valid theoretical treatment.
Table 2 The ND, c’ v, + nv, combination band ‘) Level 102s I220
E (cm-r) 63956 b’ 66280
1’2’ 1’22 1’2’
(66910) 67566 68310
1’24 1’25
69056 69813
Av (cm-‘) ,,,,c L.?‘, (630) (656) 744 -I*19, 757
a) Values given in parentheses are tentative since wavelength scans in either the photoelectron spectrometer or the mass spectrometer were unable to unambiguously locate the ND3 C’ 1’2’band due to an NDsH impurity in the former and overlapping fi state lines in the latter. b)Fromref. [16].
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Electron KE (eV) Fig. 4. The 3 + 1 MPI photoelectron spectrum of the b 1’2’level (1~430.03 nm). Production of NH: a 1‘23occurs via the overlapping e’ 1’2’ state at the three-photon level. The two peaks marked with asterisks arc unassigned and may be due to ionization of photofragments of NHs.
Volume 145, number 3
CHEMICAL PHYSICS LETTERS
4. Conclusion By employing both time-of-flight mass spectroscopy and photoelectron spectroscopy coupled with the rotational cooling of a supersonic expansion, we have been able to re-assign the 2 i- 1 MPI PES of what was formerly thought to be the NH3 b state origin. The new assignment attributes the spectrum to the c’ 1‘23combination band. Four members of the NH3 c’ 122”progression were observed, with the position of the NH3 cf 1’2’ band giving a symmetric stretch frequencyofV,(C’NH,)=3223+2cm-‘.Thesame band progression was also observed in the c’ state ofND, (v~(cl NDs)=2324k2 cm-‘). In addition, from the measured photoelectron energies arising from 2 + 1 MPI of the NH3 e’ 1’2* level, we have obtained the symmetric stretch frequency of the ground state ion (~,(%NH:)=3150flOO cm-‘). The 3+ 1 MPI PES of the correct i?j state origin displayed a single dominant photoelectron peak that could be correlated to production of NH,+ 2 2’. The appearance of a nearly exclusive 2)=0 ionizing transition via 3+ 1 MPI indicates that the fi state is a “good” Rydberg and its absence in the 2 + 1 MPI PES is due to reasons other than a strong mixing with nearby states at the intermediate resonance level.
Acknowledgement The authors wish to thank Dr. William E. Conaway and Dr. Richard N. Zare for making available their 2+ 1 MPI spectrum of NDs. This work was supported by the National Science Foundation (Grant No. CHE-8318419).
I April 1988
References [ I ] A. Sur, C.V. Ramana, W.A. Chupka and S.D. Colson, J. Chem. Phys. 84 (1986) 69. [2]M.N.R. Ashfold, B. Tutcher, B. Yang, Z.K. Jin and S.L. Anderscn, J. Chem. Phys. 87 (1987) 5105. [ 31 M.G. While, M. Seaver, W.A. Chupka and SD. Colson, Phys. Rev. Letters 49 (1982) 28. [4] J.B. Pallix, P. Chen, W.A. Chupka and S.D. Colson, J. Chem. Phys. 84 (1986) 5208. [S] S.T. Pratt, P.M. Dehmer and J.L. Dehmer, J. Chem. Phys. 85 (1986) 3379. [6] W.A. Chupka, J. Chem. Phys. 87 (1987) 1488. [7] A.P. Hickman, Phys. Rev. Letters 59 (1987) 1553. [ 81 J.B. Pallix andS.D. Colson, Chem. Phys. Letters 119 (1985) 38. [9]S.R. Long, J.T. Meek, P.J. Harrington and J.P. Reilly, J. Chem. Phys. 78 (1983) 3341. [ 10] W.E. Conaway, R.J.S. Morrisonand R.N. Zare, Chem. Phys. Letters 1I3 ( 1985) 429. [ I 1 ] R.J.S. Morrison, W.E. Conaway and R.N. Zare, Chem. Phys. Letters 113 (1985) 435. [ 121 R.J.S. Morrison, W.E. Conaway, T. Ebata and R.N. Zare, J. Chem. Phys. 84 (1986) 5527. [ 131J.B. Pallix and S.D. Colson, J. Phys. Chem. 90 ( 1986) 1499. [14]P.KruitandF.H.Read,J.Phys.E16(1983)313. [ 15lB.D. Kay and A.K. Grimley, Chem. Phys. Letters 127 (1986) 303. [ 161M.N.R. Ashfold, CL. Bennett, R.N. Dixon, P. Fielden, H. Rieley and R.J. Stickland, J. Mol. Spectry. 117 ( 1986) 2 16. [ 171J.W. Rabalais, L. Karlsson, L.O. Werme, T. Bergmark and K. Siegbahn, J. Chem. Phys. 58 (1973) 3370. [ 181G. Herzberg, Electronic spectra and electronic structure of polyatomic molecules (Van Nostrand, Princeton, 1966). [ 191 G. HenbeG Infrared and Raman spectra (Van Nostrand, Princeton, 1945). [20] J.H. Glownia, S.J. Riley, S.D. Colson and G.C. Nieman, J. Chem. Phys. 73 (1980) 4296. [21] M.N.R. Ashfold, R.N. Dixon and R.J. Stickland, Chem. Phys. 88 (1984) 463. [ 221 G.C. Nieman and S.D. Colson, J. Chem. Phys. 7 1 ( 1979) 571.
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