- Email: [email protected]
CH3− formation through predissociation of Feshbach resonances in acetaldehyde
Volume
118. number 1
CHEMICAL.
CH, FORMATION IN ACEX’ALDEHYDE
THROUGH
LETTERS
PREDLSSOCIATION
Ramer DRESSLER and Michael ALLAN Jnrrnrti de Chime Physique de I’Uniwrsrr4 CH-1700 Received
PHYSICS
Frz&urg,
OF FESHBACH
RESONANCES
SwirzerIamd
31 January 1985; in fmal form 11 April 1985
A high-resolution
anion in collisions of 6.3-7 eV electrons with acetaldehyde is reported. io ‘& of the order of lo-l9 cm’. The CH7 Ions are formed wth zero kinetic energy desplle excess energy of more than 3 eV Study of the parGaIly deulerakd CH,CDO shows Lha~ no “scrambling” of Lhe hydrogen atoms occurs prior to dissocialion. Par1 of Lhc CH; ions appear LO he metitablc boxuse of slow wbralion-induced electron detachment Assignment 01 the Feshbact r-nances involved IS &scurs6d
The dissociative
study of lhe forrna~on of Lhe CH;
attachment
cross section
is estimated
1.Introduction The formation of stable fragment anions in coIlisions of free elections with gaseous molecules is termed dissociativeattachment. The process usually proceeds via a short-hved autodetachiug state of the molecular anion, called a resonance:
intent and other methods yielding information on the electionic states of the anion, such as electron Iran5 mission spectroscopy, energy dependence of viirational and electronic excitation of the target, and threshold electron spectra. The CHF formation in election impact on acetaldeHyde.
AB+e-*(AR-)‘+A+X.
CH,CHO + e- + Cq
The study of dissociative attachment of diatomlc and small polyatomic molecules has yielded a wealth of information on autodetaching states of molecular anions and the dynamics of their unimolecular dissociation reactions [l] : Recently there has been an increasinginterest in the dissociative attachment of larger organic molecules, motivated by the potential applications in organic mass spectromew, the possibility of obtaining important thermochemical data of the fragments, and the growing need to-detemtine the properties of organic anions in connection with their +emistry_ The identification of the anion states involved in the dissociative attachment in organic molecules often presents a problem, particularly in the higherenergy range. It is therefore desirableto find prototype molecules, where the anion states involved in dissociative attachmentare identified. This may be possible in suitable cases through the combined applicatibn of the dissociative attachment exper-
has been reported with low resolution by Donnan [2]. We report here a high-resolution study which revealed sharp structure, permitting the association of the dissociative attachment band with bands of similarprofile observed in other electronjmpact experiments. These facts are helpful in elucidating the mechanism of this dissociation. In addition, the properties of the CHT fragment are of current interest [3] , particularly in connection with astrophysics [4] _The process reported here could perhaps be used to construct an intense CH3_ source to permit its further study in the laboratory;
03.30 0 Elsevier Science Publishers B.V. (North-Holland Physic Publishing Division)
0 00%2614/85/S
+ HCO,
2. Experimental Both the dissociat5veattacbmen t - t ent [S] and the electron energy-Ioss instrument [63 were described earlier. In both instruments a quashnonoenergetic 93
Volume 118, number 1
CHEMICAL PHYSICS LETTERS
0.04 eV fwhm) electron beam is prepared using a trochoidal monochromator [7] _ The beam then collides with a static gaseous sample in a target chamber. In the dissociative attachment insmment, negative ions are extracted through a slit at 90” with respect to the electron beam and are focused into a 90° cylintical condenser for ion energy analysis. The ions are then either directly detected to obtain high-resolution ionyield curves of all masses, or are passed into a commercial double-focusing magnetic sector mass filter to give lower-resolution but mass-selected curves. lrt the energyloss instrument, electrons melastically scattered at O” and lg0” are passed through two trochoidal analysers in senes and detected. The energy scale in the dissociative attachment experiment was calibrated on the O-/CO peak at 9.62 eV. In the energy-loss experiment, the energy-loss (AEj scale was calibrated on sharp features in the N2 speckum and the residual energy (E,) scale on the N- autodetachment peak at 0.07 eV [g] . The incident energy was then obtained from the known AE and Er.
12 July 1985
(about
3. Results In the negative-ion yield from acetaldehyde. we observed a structured band in the 6.3-7 eV region (fig. 1 and table 1). Using the mass filter we found this band to be due to an m/e = 15 anion, that is to CH$. Our instrument is not q-zipped ~nth an absolute pressure gauge, but a comparison of signal intensities with O-/ CO2 for a gven Penmng gauge reading indicates, that the 130s section is of the order of lo-19 crn2, which is relatively large For dissociative attachment. The measurement of the ion kinetic energy distributions revealed that within our resolution all of the CHT ions are formed with zero kinetic energy. The narrow width of the observed features suggests that Feshbach resonances may be involved. Since near threshold electron energy-loss spectra were found to be an efficient means of detecting Feshbach resonances, particularly in.organic molecules [6,9,10], we recorded several such spectia The curve obtained at 0.03 eV above threshold is also shown in fig. 1 and exhibits four peaks wluch have exact counterparts in the ion yield_ These peaks may be assigned to Feshbach resonances [l] since they remam at a fmed incident electron energy and rapidly diminish in intensity when the residual energy is in94
n
2
7 E
5 %
-WREN
E
ELECTRUNS
2 z
-----__ I
,”
I
z: d \ z z
---------_
II
I
III
I
I
6.0 INCIDENT
5.5
I
I
Illllll 6.5
7.0 ELECTRON
I
I
I
I
II
I
l
I
7.5 ENERGY/eV
Fig. 1 _Yields of negativeions andnear-threshold@‘I = 0.03 ev) elcctrorusm electron impact on acctaldchyde.The ion-yield curve was found to be due to the CHP ion (exept For some Ocun-ent above 75 ev) in a separateexperiment usinga massftiter. The energy scalesof the two curveswere calibrated independently_ creased. The additional peaks at higher energies are due to Rydberg states of the neutra: CH3CH0. In the dissociative attachment spectra of the partially deuterated molecule CH,CDO we observed exclusively the m/e = 15 CHT fragment. The fragmentation of the anion is thus not accompanied by “scrambling” of the hydrogen atoms, contrary to the situation encountered in fragmentation of certain states of the acetaldehyde cation [ 111 _ The fully deuterated mole-
Table 1 Energiesand vibrational spaciTlgsof the Feshbach~esnnances observedin the CHZ yield. AII values are in eV. The absolute energiesare accurateto +O.O4eV, energy differencesto *O.Ol eV label
Energy
Vibrational SpaFing
1 2
634 6.64
0.138 b-122
Volume 118. number 1
CHEMICAL
PHYSICS
cule CD,CDO yielded the m/e = 18 CD, fragment, albeit with a weaker intensity compared to the CHT/ CH, CHO signal. We investigated the possible existence of metastable species by applymg the tune-resolved electron energyloss spectroscopy method described recently [12]. Using a pulsed incident election beam and a delayed coIncidence detection of scattered electrons, we observed prehminary evidence for’slow (E, = O-O.15 eV) electrons ejected with a deIay in the 0.1-l ps range at the positions of the ion-yield peaks in fig. 1. We interpret the delayed electrons as arising from autodetachment of metastable CHT ions. The details of the time-resolved experiment, observation of O- and H- fragment ions at different energies, and a comparison of the ion spectm with a more detailed energy-loss study will be presented in a subsequent publication [ 131.
4. Discussion The CH, -CHO dissociation energy may be calculated from standard thermochemical data 1141 to be 3.49 eV. The election affinity of CH3 was determined to be 0.08 eV [3] _ The threshold for CHS formation is therefore 3.41 eV, which means that about 3 eV of excess energy is released in the dissociation process. Since this energy does not appear as kinetic energy, rt must be deposited as internal energy of the fragments. This may happen in the form of vibrational energy or the electronic excitation of the HCO fragment. Iarge amounts of vibrational energy may be deposited even into fragments with few internal degrees of freedom, e-g. the deposition of up to 4 eV of vibrational energy in a CO fragment with no simultaneous release of kinetic energy, found rn the 8.2 eV O-/CO, dissocrative attachment band (ref. [S] , and references therein)_ Since the HCO fragment has more vibrational degrees of freedom than CO, the deposition of energy into its vibrational modes could be even more efficient, thus absorbing virtually all the excess energy of dissociation. The formation of the 2A” first excited state of HCO is also energetically possible. An intense tiansition to the (0,9,0) level of this state was found at 2.02 eV [lS] . Since all but three vibrational levels (O+, O-, and l+ in the raninversion vibration) of CHF are autodetaching [3], only those dissociations where the excess en-
LETTERS
12 July 1985
ergy is absorbed primarily by the HCO fragment will result in a stable CHF. One would therefore expect a considerable probability of fomung the higher, autodetachmg vibrational levels of CHO _Our observation of metastable CHT suggests that at least the low-lying autodetaching vibrational levels have comparatively long lifetimes (i.e. 10-6-10-7 s as compared to the common 10-12-10-15 s lifetimes of resonances)_ This could be explained by the poor overlap between the nuclear wavefimctions of the pyramidal CH3 and the planar neutral CH, (the relevant potential curves are given in ref. [3])_ The observation of metastable CHGj may also be of interest in view of the recent theoretical investigation of vibration-induced electron detachment [16]. Two properties of Feshbach resonances may serve as a guide to their assignment. First, the vibrational profile of a Feshbach resonance is usually very similar to the profiles of the parent Rydberg state of the neutral molecule and to the grandparent state of the molecular cation [l] _ Second, characteristic energy differences are found between the Feshbach resonance and its parent and grandparent [1,17] states. Two electronic states with vibrational sticture may be discerned in our ion-yield curve. They are labeled 1 and 2 in fig. 1. The close resemblance of both band profiles with the band profile of the first band in the photoelectron spectrum [18] point to the acetaldehyde cation ground state n-l % 2A’ as the grandparent of bo*& Feshbach resonances. The parent state of the resonance 1 is presumably the lowest Rydberg state of acetaldehyde, the n + 3s state located at 6.824 eV [19] _(This state is also visible as a sharp peak at En., = 6.824 + 0.03 = 6.854 eV in the upper curve in fig. 1) This results in an energy difference of 0.48 eV between the resonance and its parent state, a value consistent with the 03-0.5 eV cited as the usual electron afkity of an s-type Rydberg state to form an s2 resonance [17] . Resonance 1 is thus assigned to the n-13s2 state. Resonance 2 could bc the n-13s3p state, in analogy with the assignment suggested for similar resonances in the rare gases or the hydrogen halides 1203 _ The CHF fragment is fcmned by predissociation of the Feshbach resonances. Broad o* shape resonances appear to be present in the 2-10 eV region in virtually all organic molecules [Zl] and could provide the repulsive surface involved in the predissocration. 95
Volume 118, number1
CHEMICALPHYSICSLETTERS
After the completion of this work we learned tbat a closeiy related pretiociation of a Fesbbach resonance in NH3 was observed by Burrow and Sticklett 1221 -
Adrnowledgement The authors wish to express their sincere appreciation to Professor E. Haselbach for his continuing support and encouragement in the present work. We are indebted to E. Brosi, M. Gremaud and P.-H. Chassot for help in the construction of the apparata. We thank PD. Burrow (Nebraska) for important comments on the manuscript. This work is part of project No. 2.2190.84 of the Schweizerischer Nationalfonds zur Forderung der wissenschafthchen Forschung.
References [l] [2] [3]
GJ Schulz, Rev. Mod. Phys. 45 (1973) 423. FH. Dorman, J Chem. Phys. 4-4 (1966) 3856. G.B. Ellison, PC. Erg&&g and W.C Lineberger. J. Am Chem. Sot. 100 (1978) 2556 [4] V. Splrko and P.R Bunker, J. MoL Spectcy. 95 (1982) 226_ [5] R. Dressler and M. A&n, Chem. Phyr 92 (1985) 449. [6] M. AJlan, Helv. Chlm. Acta 65 (1982) 2008. [7] A. Stamatovic’and GJ Schulz, Rev Sci Insu. 39 (1968) 1752; 41 (1970) 423.
96
[S]
12 July 1985
D_ Spence and PD Borrow, J. Phys. B12 (1978) L179; J_ Mazeau, F. Grestmu. R,‘. Hall and A. Huetz. J. Phys. Bll(1978) U57. 191 M. A.&n. J. Chem. Phvs. 80 (1984) 6020. D. Spen& J. Chem. Phys. 74 (198i) 3898. H-W. Joehims. W. L&r and H. Baumgtiel. Chem. Phys Letters 54 (1978) 594; G J. Fkanick and TS_ E.ichelberger TV. J. Chem. Phys 74 (1981) 6692. 1121 M. Allan. Chem. Phys. 81<1983) 235 1131 R. Dresskr and M. All&. in preparation 1141 H.M. Rosenstock,K. Dmxl. B.W. Steiner and J.T. Herron, J. Phys Chem. Ref. Data 6. SuppI. l(197.7); R. Bombach. J--P. Stadehnann and J_ Vogt. Chem_ Phys 60 (1981) 293. WI J-WC. Johns, SH. Riddle and DA. Rammy, Discussions Faraday Sot 35 (1963) 90. RA. Kendall and J. Sirnons, J. Am. 1163 PK. Admya, Chem. Sot. 106 (1984) 3402 1171 D. Spence. J. Chem. Phys. 66 (1977) 669. J. Chem. Phys 64 WI T. Cvita& H. GUsten and L. -6. (1976) 2549, and Iefcrences therein. r191 M.D. Robin, Higher excited states of polyatomic molecules (Academic Press, New York, 1975) p_ 75; T S. Eichelberger lV and GJ. F&nick, J. Chem. Phys 74 (1981) 5962. 1201 L. Sanche and GJ. Schulz, Phys. Rev_ ti (1972) 1672; D. Spence and T. Noguchi, J. Chem. Phys. 63 (1975) 505; D. Spence. Phyr Rev_ A15 (1977) 883. and SF. Wang, J. Chem. WI 1-C. Walker, A Stsatovid Phys 69 (1978) 5532; M. Altan, Chem. Phys 84 (1984) 311; 86 (1984) 303. 1221 P.D. Burrow and K.L. Stricklett. Bull_ Am. Phys. SOL 30 (1985) 148.
118. number 1
CHEMICAL.
CH, FORMATION IN ACEX’ALDEHYDE
THROUGH
LETTERS
PREDLSSOCIATION
Ramer DRESSLER and Michael ALLAN Jnrrnrti de Chime Physique de I’Uniwrsrr4 CH-1700 Received
PHYSICS
Frz&urg,
OF FESHBACH
RESONANCES
SwirzerIamd
31 January 1985; in fmal form 11 April 1985
A high-resolution
anion in collisions of 6.3-7 eV electrons with acetaldehyde is reported. io ‘& of the order of lo-l9 cm’. The CH7 Ions are formed wth zero kinetic energy desplle excess energy of more than 3 eV Study of the parGaIly deulerakd CH,CDO shows Lha~ no “scrambling” of Lhe hydrogen atoms occurs prior to dissocialion. Par1 of Lhc CH; ions appear LO he metitablc boxuse of slow wbralion-induced electron detachment Assignment 01 the Feshbact r-nances involved IS &scurs6d
The dissociative
study of lhe forrna~on of Lhe CH;
attachment
cross section
is estimated
1.Introduction The formation of stable fragment anions in coIlisions of free elections with gaseous molecules is termed dissociativeattachment. The process usually proceeds via a short-hved autodetachiug state of the molecular anion, called a resonance:
intent and other methods yielding information on the electionic states of the anion, such as electron Iran5 mission spectroscopy, energy dependence of viirational and electronic excitation of the target, and threshold electron spectra. The CHF formation in election impact on acetaldeHyde.
AB+e-*(AR-)‘+A+X.
CH,CHO + e- + Cq
The study of dissociative attachment of diatomlc and small polyatomic molecules has yielded a wealth of information on autodetaching states of molecular anions and the dynamics of their unimolecular dissociation reactions [l] : Recently there has been an increasinginterest in the dissociative attachment of larger organic molecules, motivated by the potential applications in organic mass spectromew, the possibility of obtaining important thermochemical data of the fragments, and the growing need to-detemtine the properties of organic anions in connection with their +emistry_ The identification of the anion states involved in the dissociative attachment in organic molecules often presents a problem, particularly in the higherenergy range. It is therefore desirableto find prototype molecules, where the anion states involved in dissociative attachmentare identified. This may be possible in suitable cases through the combined applicatibn of the dissociative attachment exper-
has been reported with low resolution by Donnan [2]. We report here a high-resolution study which revealed sharp structure, permitting the association of the dissociative attachment band with bands of similarprofile observed in other electronjmpact experiments. These facts are helpful in elucidating the mechanism of this dissociation. In addition, the properties of the CHT fragment are of current interest [3] , particularly in connection with astrophysics [4] _The process reported here could perhaps be used to construct an intense CH3_ source to permit its further study in the laboratory;
03.30 0 Elsevier Science Publishers B.V. (North-Holland Physic Publishing Division)
0 00%2614/85/S
+ HCO,
2. Experimental Both the dissociat5veattacbmen t - t ent [S] and the electron energy-Ioss instrument [63 were described earlier. In both instruments a quashnonoenergetic 93
Volume 118, number 1
CHEMICAL PHYSICS LETTERS
0.04 eV fwhm) electron beam is prepared using a trochoidal monochromator [7] _ The beam then collides with a static gaseous sample in a target chamber. In the dissociative attachment insmment, negative ions are extracted through a slit at 90” with respect to the electron beam and are focused into a 90° cylintical condenser for ion energy analysis. The ions are then either directly detected to obtain high-resolution ionyield curves of all masses, or are passed into a commercial double-focusing magnetic sector mass filter to give lower-resolution but mass-selected curves. lrt the energyloss instrument, electrons melastically scattered at O” and lg0” are passed through two trochoidal analysers in senes and detected. The energy scale in the dissociative attachment experiment was calibrated on the O-/CO peak at 9.62 eV. In the energy-loss experiment, the energy-loss (AEj scale was calibrated on sharp features in the N2 speckum and the residual energy (E,) scale on the N- autodetachment peak at 0.07 eV [g] . The incident energy was then obtained from the known AE and Er.
12 July 1985
(about
3. Results In the negative-ion yield from acetaldehyde. we observed a structured band in the 6.3-7 eV region (fig. 1 and table 1). Using the mass filter we found this band to be due to an m/e = 15 anion, that is to CH$. Our instrument is not q-zipped ~nth an absolute pressure gauge, but a comparison of signal intensities with O-/ CO2 for a gven Penmng gauge reading indicates, that the 130s section is of the order of lo-19 crn2, which is relatively large For dissociative attachment. The measurement of the ion kinetic energy distributions revealed that within our resolution all of the CHT ions are formed with zero kinetic energy. The narrow width of the observed features suggests that Feshbach resonances may be involved. Since near threshold electron energy-loss spectra were found to be an efficient means of detecting Feshbach resonances, particularly in.organic molecules [6,9,10], we recorded several such spectia The curve obtained at 0.03 eV above threshold is also shown in fig. 1 and exhibits four peaks wluch have exact counterparts in the ion yield_ These peaks may be assigned to Feshbach resonances [l] since they remam at a fmed incident electron energy and rapidly diminish in intensity when the residual energy is in94
n
2
7 E
5 %
-WREN
E
ELECTRUNS
2 z
-----__ I
,”
I
z: d \ z z
---------_
II
I
III
I
I
6.0 INCIDENT
5.5
I
I
Illllll 6.5
7.0 ELECTRON
I
I
I
I
II
I
l
I
7.5 ENERGY/eV
Fig. 1 _Yields of negativeions andnear-threshold@‘I = 0.03 ev) elcctrorusm electron impact on acctaldchyde.The ion-yield curve was found to be due to the CHP ion (exept For some Ocun-ent above 75 ev) in a separateexperiment usinga massftiter. The energy scalesof the two curveswere calibrated independently_ creased. The additional peaks at higher energies are due to Rydberg states of the neutra: CH3CH0. In the dissociative attachment spectra of the partially deuterated molecule CH,CDO we observed exclusively the m/e = 15 CHT fragment. The fragmentation of the anion is thus not accompanied by “scrambling” of the hydrogen atoms, contrary to the situation encountered in fragmentation of certain states of the acetaldehyde cation [ 111 _ The fully deuterated mole-
Table 1 Energiesand vibrational spaciTlgsof the Feshbach~esnnances observedin the CHZ yield. AII values are in eV. The absolute energiesare accurateto +O.O4eV, energy differencesto *O.Ol eV label
Energy
Vibrational SpaFing
1 2
634 6.64
0.138 b-122
Volume 118. number 1
CHEMICAL
PHYSICS
cule CD,CDO yielded the m/e = 18 CD, fragment, albeit with a weaker intensity compared to the CHT/ CH, CHO signal. We investigated the possible existence of metastable species by applymg the tune-resolved electron energyloss spectroscopy method described recently [12]. Using a pulsed incident election beam and a delayed coIncidence detection of scattered electrons, we observed prehminary evidence for’slow (E, = O-O.15 eV) electrons ejected with a deIay in the 0.1-l ps range at the positions of the ion-yield peaks in fig. 1. We interpret the delayed electrons as arising from autodetachment of metastable CHT ions. The details of the time-resolved experiment, observation of O- and H- fragment ions at different energies, and a comparison of the ion spectm with a more detailed energy-loss study will be presented in a subsequent publication [ 131.
4. Discussion The CH, -CHO dissociation energy may be calculated from standard thermochemical data 1141 to be 3.49 eV. The election affinity of CH3 was determined to be 0.08 eV [3] _ The threshold for CHS formation is therefore 3.41 eV, which means that about 3 eV of excess energy is released in the dissociation process. Since this energy does not appear as kinetic energy, rt must be deposited as internal energy of the fragments. This may happen in the form of vibrational energy or the electronic excitation of the HCO fragment. Iarge amounts of vibrational energy may be deposited even into fragments with few internal degrees of freedom, e-g. the deposition of up to 4 eV of vibrational energy in a CO fragment with no simultaneous release of kinetic energy, found rn the 8.2 eV O-/CO, dissocrative attachment band (ref. [S] , and references therein)_ Since the HCO fragment has more vibrational degrees of freedom than CO, the deposition of energy into its vibrational modes could be even more efficient, thus absorbing virtually all the excess energy of dissociation. The formation of the 2A” first excited state of HCO is also energetically possible. An intense tiansition to the (0,9,0) level of this state was found at 2.02 eV [lS] . Since all but three vibrational levels (O+, O-, and l+ in the raninversion vibration) of CHF are autodetaching [3], only those dissociations where the excess en-
LETTERS
12 July 1985
ergy is absorbed primarily by the HCO fragment will result in a stable CHF. One would therefore expect a considerable probability of fomung the higher, autodetachmg vibrational levels of CHO _Our observation of metastable CHT suggests that at least the low-lying autodetaching vibrational levels have comparatively long lifetimes (i.e. 10-6-10-7 s as compared to the common 10-12-10-15 s lifetimes of resonances)_ This could be explained by the poor overlap between the nuclear wavefimctions of the pyramidal CH3 and the planar neutral CH, (the relevant potential curves are given in ref. [3])_ The observation of metastable CHGj may also be of interest in view of the recent theoretical investigation of vibration-induced electron detachment [16]. Two properties of Feshbach resonances may serve as a guide to their assignment. First, the vibrational profile of a Feshbach resonance is usually very similar to the profiles of the parent Rydberg state of the neutral molecule and to the grandparent state of the molecular cation [l] _ Second, characteristic energy differences are found between the Feshbach resonance and its parent and grandparent [1,17] states. Two electronic states with vibrational sticture may be discerned in our ion-yield curve. They are labeled 1 and 2 in fig. 1. The close resemblance of both band profiles with the band profile of the first band in the photoelectron spectrum [18] point to the acetaldehyde cation ground state n-l % 2A’ as the grandparent of bo*& Feshbach resonances. The parent state of the resonance 1 is presumably the lowest Rydberg state of acetaldehyde, the n + 3s state located at 6.824 eV [19] _(This state is also visible as a sharp peak at En., = 6.824 + 0.03 = 6.854 eV in the upper curve in fig. 1) This results in an energy difference of 0.48 eV between the resonance and its parent state, a value consistent with the 03-0.5 eV cited as the usual electron afkity of an s-type Rydberg state to form an s2 resonance [17] . Resonance 1 is thus assigned to the n-13s2 state. Resonance 2 could bc the n-13s3p state, in analogy with the assignment suggested for similar resonances in the rare gases or the hydrogen halides 1203 _ The CHF fragment is fcmned by predissociation of the Feshbach resonances. Broad o* shape resonances appear to be present in the 2-10 eV region in virtually all organic molecules [Zl] and could provide the repulsive surface involved in the predissocration. 95
Volume 118, number1
CHEMICALPHYSICSLETTERS
After the completion of this work we learned tbat a closeiy related pretiociation of a Fesbbach resonance in NH3 was observed by Burrow and Sticklett 1221 -
Adrnowledgement The authors wish to express their sincere appreciation to Professor E. Haselbach for his continuing support and encouragement in the present work. We are indebted to E. Brosi, M. Gremaud and P.-H. Chassot for help in the construction of the apparata. We thank PD. Burrow (Nebraska) for important comments on the manuscript. This work is part of project No. 2.2190.84 of the Schweizerischer Nationalfonds zur Forderung der wissenschafthchen Forschung.
References [l] [2] [3]
GJ Schulz, Rev. Mod. Phys. 45 (1973) 423. FH. Dorman, J Chem. Phys. 4-4 (1966) 3856. G.B. Ellison, PC. Erg&&g and W.C Lineberger. J. Am Chem. Sot. 100 (1978) 2556 [4] V. Splrko and P.R Bunker, J. MoL Spectcy. 95 (1982) 226_ [5] R. Dressler and M. A&n, Chem. Phyr 92 (1985) 449. [6] M. AJlan, Helv. Chlm. Acta 65 (1982) 2008. [7] A. Stamatovic’and GJ Schulz, Rev Sci Insu. 39 (1968) 1752; 41 (1970) 423.
96
[S]
12 July 1985
D_ Spence and PD Borrow, J. Phys. B12 (1978) L179; J_ Mazeau, F. Grestmu. R,‘. Hall and A. Huetz. J. Phys. Bll(1978) U57. 191 M. A.&n. J. Chem. Phvs. 80 (1984) 6020. D. Spen& J. Chem. Phys. 74 (198i) 3898. H-W. Joehims. W. L&r and H. Baumgtiel. Chem. Phys Letters 54 (1978) 594; G J. Fkanick and TS_ E.ichelberger TV. J. Chem. Phys 74 (1981) 6692. 1121 M. Allan. Chem. Phys. 81<1983) 235 1131 R. Dresskr and M. All&. in preparation 1141 H.M. Rosenstock,K. Dmxl. B.W. Steiner and J.T. Herron, J. Phys Chem. Ref. Data 6. SuppI. l(197.7); R. Bombach. J--P. Stadehnann and J_ Vogt. Chem_ Phys 60 (1981) 293. WI J-WC. Johns, SH. Riddle and DA. Rammy, Discussions Faraday Sot 35 (1963) 90. RA. Kendall and J. Sirnons, J. Am. 1163 PK. Admya, Chem. Sot. 106 (1984) 3402 1171 D. Spence. J. Chem. Phys. 66 (1977) 669. J. Chem. Phys 64 WI T. Cvita& H. GUsten and L. -6. (1976) 2549, and Iefcrences therein. r191 M.D. Robin, Higher excited states of polyatomic molecules (Academic Press, New York, 1975) p_ 75; T S. Eichelberger lV and GJ. F&nick, J. Chem. Phys 74 (1981) 5962. 1201 L. Sanche and GJ. Schulz, Phys. Rev_ ti (1972) 1672; D. Spence and T. Noguchi, J. Chem. Phys. 63 (1975) 505; D. Spence. Phyr Rev_ A15 (1977) 883. and SF. Wang, J. Chem. WI 1-C. Walker, A Stsatovid Phys 69 (1978) 5532; M. Altan, Chem. Phys 84 (1984) 311; 86 (1984) 303. 1221 P.D. Burrow and K.L. Stricklett. Bull_ Am. Phys. SOL 30 (1985) 148.