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The photoelectron spectra of the OH and OD radicals
CHEMICAL PHYSICS LETTERS
Volume 45, number 3
1 February
1977
THE PHOTOELECTRON SPECTRA OF THE OH AND OD RADICALS S. KATSUMATAS and D.R. LLOYD Ozemistry Department, Received 21 October
The University. Eirmingham B1.5 2TT, UK
1976
He(I) photoelectron spectra of the hydroxyl radical show ionizations corresponding to the production of OH* (CID+) in the states X32-and ‘A at ionization potentials of 13.01 eV and 15.20 eV respectively. The excitation energy of the *A state is in very good agreement with that predicted by calculations and experiment; vibrational spacings in the X ‘Z- state agree with emission spectroscopic data.
2. Experimental
I. Introduction The hydroxyl
radical
has the ground
electronic
state 211, arising from the configuration lo22u23a2 1n3, and the hydroxyl cation ground state 3Z- arises from the configuration lo22u23u2in2. Although both these species have been studied intensively and many excited states of OH are known [ 1] , experimental measurements of the ionization potential (IP) connecting these two ground states are sparse. Values of 13.18 eV [2] and 13.53 eV [3] have been reported, and a frequently referenced [4,5] value of 13.36 eV appears to be an average of these two. A value of 13.0 eV has been mentioned by Price and co-workers [6]. A detailed theoretical analysis of electron correlation effects recommends a probable IP of 13.0 eV [5]. Calculations of the excited states of OH’ have been reported [7] , but only the A 311 excited state has been observed directly. However, a detailed analysis of perturbations in the A 311+ X 3IZemission shows the presence of a lZ+ state at a similar energy to the A 311 and a ‘A state at lower energy [8]. We have studied the He(I) photoelectron spectra of OH and OD to obtain further experimental information.
* Permanent address: Physical Chemistry Laboratory, Institute of Applied Electricity, Hokkaido University, Sapporo, Japan.
Spectra were obtained on an instrument with a hemispherical electrostatic deflection analyser !9] _ The ionization region was evacuated with a 4” diffusion pump and liquid nitrogen trap; pressure in the pumping line in these experiments was ==10M4 torr. Spectra were calibrated with the ionizations of HZO, H2 and H. The OH radical was produced by the reaction of NO2 with H atoms produced in a microwave discharge in H2 [lo] . The discharge tube was constructed from Pyrex glass and is shown in fig. 1; all inside surfaces were coated with phosphoric acid [I l] _ A 1 mm diameter pinhole maintains a pressure differential between the discharge and ionization regions. The inner and outer coaxial tubes are connected by six 1 mm pinholes about 20 mm from the end, and the end was set about 10 mm away from the He(I) photon beam. H atoms were produced by a 2450 MHz discharge, at up to 150 W power, in pure Hz with no diluent gas. A variety of other discharge
Fig. 1. Discharge flow tube used for the production radicals.
of OH
519
CHEMICALPHYSICSLETTERS
Volume 43, number 3
and flow configurdrions have been tried, but this systern p:oduced the greatest intensity of OH spectra. The same system was used for the OD spectra. Attempts to produce the OH spectrum from discharges in HZ0 were unsuccessful. D2 was prepared by reaction of D,O with metallic calcium, and NO2 by pyrolYsis of FIJ(NO,)~; H2 was standard cylinder quality-
3. Results and discussion
Operation of the discharge in Hz produced the well characterised H atom spectrum [ 121; the meximum intensity of the 13.6 eV photoelectron line was Gghtly greater than that of the H2 band maximum when corrected for analyser transmission. No vibrationally excited Hz was present [ 131. The spectrum produced by adding NO2 to the flowing discharge system is shown in fig. 2. The amount of NO2 has been set to optimise the intensity of the bands believed to be due to OH, and under these conditions the NO, spectrum is very weak. Higher pressures of NO2 reduce the intensity of both H and OH spectra, and ultimately the spectrum of NO, [ 141 appears. In addition to H2 and H, bands of NO, 0, and HZ0 [ 1S-181 are present, presumably from the reactions llO] : H+NO,+OH+-NO, 0H+OH-+H20+0, 0+OH+02+H.
I
12
0
13
1‘ IONIZATION
2. The He(I) photoelectron NO2 to a flow of H atoms in
POTENTIAL
I
I
15
16
lob’1
spectrum obtamed by adding the apparatus of rig. 1. The bands marked with arrows are asslgned to ionization of OH radicals; weaker bands are discussed in the text. Gig.
520
1 February 1977
The weak bands at 13.80 and 14.05 eV are believed to be due to CO2 and CO from reaction with carbon containing contaminants; they decrease slowly with time but are particularly strong when a discharge is run in 02. The weak ionization around 14.7 eV is due mainly to ionization of NO by the He I /3photons. Two previously unreported bands, at IP of 13.01 eV and 15.20 eV, are arrowed on fig. 2. The relative intensity of these two bands remains constant as experimental conditions change, and their intensity decreases relative to the H20 ionization as the pressure in the tube is increased, and in discharge systems with a longer path length between the mixing and ionization regions. We identify these bands as being due to ionization of OH to the states X 3Z- and lA of OH+. The first IP is in very good agreement with the value of 13.0 eV recommended from calculations [5] . The 13.01 eV band is accompanied by a weak satellite at 13.38 eV which has some weak interference from components of the 02 and Hz0 ionizations. Spectra of H20 and 0, were run under similar conditions but without the discharge running, and a point-by-point subtraction was carried out, yielding the spectrum shown in the upper half of fig. 3a. The satellite appears to be a vibrational component; to check this we obtained the OD spectrum shown in fig. 3b. Vibrational spacings from this work are shown in table 1 and are in satisfactory agreement with values for the X 3Zstate of OH+ and OD” obtained from emission spectra [8] . Estimates of the relative intensities of the 1-O components are also given in table 1. A search was made for vibrational components of the IA state, but as can be seen from fig. 2 there is substantial interference in this region from bands of NO and H, (D2). The position of the A 3fl and ‘IZf states can be predicted from the emission spectral data [8] but these are in a region where NO has a strong and complex spectrum, and no evidence for these states could be found. The excitation energy of lA above X 3Z- is 2.19 eV according to our measurement, which compares well with a value from calculations of 2.08 eV 171. The only previous experimental evidence for this lA state is a perturbation in the A 3fI + X 3C- emission cf OD”, and since the perturbing level is a high vibrational level of ‘A the vibrational numbering was not certain. Merer et al. give alternative values for TO (IA) of 2.175 eV if u = 6 or
2.397 eV if u = 5 [8]. Our value of 2.19 eV allows a
CHEMICAL PHYSICS LETTERS
Volume 45, number 3
1 February
1977
(b)
(a)
9 ,
J !LjL
.,Ixx.‘l. -’
120
125
--_.-
_____
130
IONIZATION
135
POTENTIAL
-
IeV)
Fig. 3. The first ionization of the hydroxyl radical obtained by subtracting the spectra of water (broken line) and oxygen from the spectrum obtained under the conditions of fig. 2 (full line). The lower half shows the experimental spectra and the upper half the difference spectra (a) OH, (b) OD.
Table 1 Observed O-1 vibrational
spacing and the intensity O-l
OH(X 3B-) OD(X3z-) a) Brackets show the O-l vibratronal
ratio in the ground state of OH+ and OD+ radtcal
vibrational
Intensity v=O:u=l
spacing (cm-t)
this work
ref. [S]
2950 + 50 2260 + 50
2956 135691 a) 2185 [2632]
ratio
l:O.ls LO.25
spacmg of the OH and OD ground state taken from ref. [ 11.
confirmation of their preference for u = 6, and the agreement is within the limits of our experimental error.
References [l] G. Herzberg, Spectra of diatomic molecules (Van NOstrand, Princeton, 1950). [2] S.N. Foner and R.L. Hudson, J. Chem. Phys. 25 (1956;
602. (31 L.P. Lindeman and J.C. Guffy, J. Chem. Phys. 29 (1958)
Acknowledgement
We thank the S.R.C. of Great Britain for support, including
a fellowship
(SK.),
and Dr. S. Leach for
discussions and for drawing our attention to ref. [8] .
247. [4] P.E. Cade and W.M. Huo, J. Chem. Phys. 47 (1967) 614.
[5] W. Meyer, Theoret. ChIm. Acta 35 (1974) 277. [6] W.C. Price, T.R. Passmore and D.ht_ Roesster, Discussions Faraday Sot. 35 (1963) 201.
521
Volume 45. number 3
CHEMICAL PHYSICS LETTERS
(71 H.P.D. Liu and G. Verhaegen, Intern. J. Quantum Chem. 5 (1971) 103. (81 A-l. Merer, D.N. hlalm, R.W. Martin, M. Horani and J. Rostas,Can. J. Phys. 53 (1975) 251. [S] D.R. Lloyd, P.J. Roberts and I.H. Htllier, J. Chcm. Sot. Faraday Trans. 11 71 (1975) 496; PJ. Roberts, Thesis. University of Birmingham (1974). [lo] K.R. German and R.N. Zare, Phys. Rev. 186 (1969) 9. [ 111 E.A. Ogryzlo, Can. J. Chem. 39 (1961) 2556. 1121 N. Jonathan, A. Morris, D.J. Smith and K.J. Ross,Chem. Phys. Letters 7 (1970) 497. (131 J. Dyke, N. Jonathan, A. Morris and T. Sears, J. Chem. Sot. Faraday Trans. II 72 (1976) 597.
522
1 February
1977
[14] C.R. Bruadie. D. Neumann, WC. Price, D. Evans, A.W. Potts ant D.G. Streets, J. Chem. Phys. 53 (1970) 705. 115 ] D.W. Tu:-ner. C. Baker, A.D. Baker and C.R. Brundle, Molecuk r photoelectron spectroscopy (Wiley-Interscience, New York. 1970). 1161 0. Edqvist, E. Lindbolm, LX. Selin, H. SjBgrcn and L. Asbrink, Arkiv Fysik 40 (1970) 439. [I7] 0. Edqvist, E. Lindholm. L.E. Selin and L. Asbrink, Phys. Scripta 1 (1970) 25. [ 18) L. Karlsson, L. Mat&on. R. Jadrny, R.G. Albridge, S. Pinchas, T. Bergmark and K. Siegbahn, J. Chem. Phys. 62 (1975) 4745.
Volume 45, number 3
1 February
1977
THE PHOTOELECTRON SPECTRA OF THE OH AND OD RADICALS S. KATSUMATAS and D.R. LLOYD Ozemistry Department, Received 21 October
The University. Eirmingham B1.5 2TT, UK
1976
He(I) photoelectron spectra of the hydroxyl radical show ionizations corresponding to the production of OH* (CID+) in the states X32-and ‘A at ionization potentials of 13.01 eV and 15.20 eV respectively. The excitation energy of the *A state is in very good agreement with that predicted by calculations and experiment; vibrational spacings in the X ‘Z- state agree with emission spectroscopic data.
2. Experimental
I. Introduction The hydroxyl
radical
has the ground
electronic
state 211, arising from the configuration lo22u23a2 1n3, and the hydroxyl cation ground state 3Z- arises from the configuration lo22u23u2in2. Although both these species have been studied intensively and many excited states of OH are known [ 1] , experimental measurements of the ionization potential (IP) connecting these two ground states are sparse. Values of 13.18 eV [2] and 13.53 eV [3] have been reported, and a frequently referenced [4,5] value of 13.36 eV appears to be an average of these two. A value of 13.0 eV has been mentioned by Price and co-workers [6]. A detailed theoretical analysis of electron correlation effects recommends a probable IP of 13.0 eV [5]. Calculations of the excited states of OH’ have been reported [7] , but only the A 311 excited state has been observed directly. However, a detailed analysis of perturbations in the A 311+ X 3IZemission shows the presence of a lZ+ state at a similar energy to the A 311 and a ‘A state at lower energy [8]. We have studied the He(I) photoelectron spectra of OH and OD to obtain further experimental information.
* Permanent address: Physical Chemistry Laboratory, Institute of Applied Electricity, Hokkaido University, Sapporo, Japan.
Spectra were obtained on an instrument with a hemispherical electrostatic deflection analyser !9] _ The ionization region was evacuated with a 4” diffusion pump and liquid nitrogen trap; pressure in the pumping line in these experiments was ==10M4 torr. Spectra were calibrated with the ionizations of HZO, H2 and H. The OH radical was produced by the reaction of NO2 with H atoms produced in a microwave discharge in H2 [lo] . The discharge tube was constructed from Pyrex glass and is shown in fig. 1; all inside surfaces were coated with phosphoric acid [I l] _ A 1 mm diameter pinhole maintains a pressure differential between the discharge and ionization regions. The inner and outer coaxial tubes are connected by six 1 mm pinholes about 20 mm from the end, and the end was set about 10 mm away from the He(I) photon beam. H atoms were produced by a 2450 MHz discharge, at up to 150 W power, in pure Hz with no diluent gas. A variety of other discharge
Fig. 1. Discharge flow tube used for the production radicals.
of OH
519
CHEMICALPHYSICSLETTERS
Volume 43, number 3
and flow configurdrions have been tried, but this systern p:oduced the greatest intensity of OH spectra. The same system was used for the OD spectra. Attempts to produce the OH spectrum from discharges in HZ0 were unsuccessful. D2 was prepared by reaction of D,O with metallic calcium, and NO2 by pyrolYsis of FIJ(NO,)~; H2 was standard cylinder quality-
3. Results and discussion
Operation of the discharge in Hz produced the well characterised H atom spectrum [ 121; the meximum intensity of the 13.6 eV photoelectron line was Gghtly greater than that of the H2 band maximum when corrected for analyser transmission. No vibrationally excited Hz was present [ 131. The spectrum produced by adding NO2 to the flowing discharge system is shown in fig. 2. The amount of NO2 has been set to optimise the intensity of the bands believed to be due to OH, and under these conditions the NO, spectrum is very weak. Higher pressures of NO2 reduce the intensity of both H and OH spectra, and ultimately the spectrum of NO, [ 141 appears. In addition to H2 and H, bands of NO, 0, and HZ0 [ 1S-181 are present, presumably from the reactions llO] : H+NO,+OH+-NO, 0H+OH-+H20+0, 0+OH+02+H.
I
12
0
13
1‘ IONIZATION
2. The He(I) photoelectron NO2 to a flow of H atoms in
POTENTIAL
I
I
15
16
lob’1
spectrum obtamed by adding the apparatus of rig. 1. The bands marked with arrows are asslgned to ionization of OH radicals; weaker bands are discussed in the text. Gig.
520
1 February 1977
The weak bands at 13.80 and 14.05 eV are believed to be due to CO2 and CO from reaction with carbon containing contaminants; they decrease slowly with time but are particularly strong when a discharge is run in 02. The weak ionization around 14.7 eV is due mainly to ionization of NO by the He I /3photons. Two previously unreported bands, at IP of 13.01 eV and 15.20 eV, are arrowed on fig. 2. The relative intensity of these two bands remains constant as experimental conditions change, and their intensity decreases relative to the H20 ionization as the pressure in the tube is increased, and in discharge systems with a longer path length between the mixing and ionization regions. We identify these bands as being due to ionization of OH to the states X 3Z- and lA of OH+. The first IP is in very good agreement with the value of 13.0 eV recommended from calculations [5] . The 13.01 eV band is accompanied by a weak satellite at 13.38 eV which has some weak interference from components of the 02 and Hz0 ionizations. Spectra of H20 and 0, were run under similar conditions but without the discharge running, and a point-by-point subtraction was carried out, yielding the spectrum shown in the upper half of fig. 3a. The satellite appears to be a vibrational component; to check this we obtained the OD spectrum shown in fig. 3b. Vibrational spacings from this work are shown in table 1 and are in satisfactory agreement with values for the X 3Zstate of OH+ and OD” obtained from emission spectra [8] . Estimates of the relative intensities of the 1-O components are also given in table 1. A search was made for vibrational components of the IA state, but as can be seen from fig. 2 there is substantial interference in this region from bands of NO and H, (D2). The position of the A 3fl and ‘IZf states can be predicted from the emission spectral data [8] but these are in a region where NO has a strong and complex spectrum, and no evidence for these states could be found. The excitation energy of lA above X 3Z- is 2.19 eV according to our measurement, which compares well with a value from calculations of 2.08 eV 171. The only previous experimental evidence for this lA state is a perturbation in the A 3fI + X 3C- emission cf OD”, and since the perturbing level is a high vibrational level of ‘A the vibrational numbering was not certain. Merer et al. give alternative values for TO (IA) of 2.175 eV if u = 6 or
2.397 eV if u = 5 [8]. Our value of 2.19 eV allows a
CHEMICAL PHYSICS LETTERS
Volume 45, number 3
1 February
1977
(b)
(a)
9 ,
J !LjL
.,Ixx.‘l. -’
120
125
--_.-
_____
130
IONIZATION
135
POTENTIAL
-
IeV)
Fig. 3. The first ionization of the hydroxyl radical obtained by subtracting the spectra of water (broken line) and oxygen from the spectrum obtained under the conditions of fig. 2 (full line). The lower half shows the experimental spectra and the upper half the difference spectra (a) OH, (b) OD.
Table 1 Observed O-1 vibrational
spacing and the intensity O-l
OH(X 3B-) OD(X3z-) a) Brackets show the O-l vibratronal
ratio in the ground state of OH+ and OD+ radtcal
vibrational
Intensity v=O:u=l
spacing (cm-t)
this work
ref. [S]
2950 + 50 2260 + 50
2956 135691 a) 2185 [2632]
ratio
l:O.ls LO.25
spacmg of the OH and OD ground state taken from ref. [ 11.
confirmation of their preference for u = 6, and the agreement is within the limits of our experimental error.
References [l] G. Herzberg, Spectra of diatomic molecules (Van NOstrand, Princeton, 1950). [2] S.N. Foner and R.L. Hudson, J. Chem. Phys. 25 (1956;
602. (31 L.P. Lindeman and J.C. Guffy, J. Chem. Phys. 29 (1958)
Acknowledgement
We thank the S.R.C. of Great Britain for support, including
a fellowship
(SK.),
and Dr. S. Leach for
discussions and for drawing our attention to ref. [8] .
247. [4] P.E. Cade and W.M. Huo, J. Chem. Phys. 47 (1967) 614.
[5] W. Meyer, Theoret. ChIm. Acta 35 (1974) 277. [6] W.C. Price, T.R. Passmore and D.ht_ Roesster, Discussions Faraday Sot. 35 (1963) 201.
521
Volume 45. number 3
CHEMICAL PHYSICS LETTERS
(71 H.P.D. Liu and G. Verhaegen, Intern. J. Quantum Chem. 5 (1971) 103. (81 A-l. Merer, D.N. hlalm, R.W. Martin, M. Horani and J. Rostas,Can. J. Phys. 53 (1975) 251. [S] D.R. Lloyd, P.J. Roberts and I.H. Htllier, J. Chcm. Sot. Faraday Trans. 11 71 (1975) 496; PJ. Roberts, Thesis. University of Birmingham (1974). [lo] K.R. German and R.N. Zare, Phys. Rev. 186 (1969) 9. [ 111 E.A. Ogryzlo, Can. J. Chem. 39 (1961) 2556. 1121 N. Jonathan, A. Morris, D.J. Smith and K.J. Ross,Chem. Phys. Letters 7 (1970) 497. (131 J. Dyke, N. Jonathan, A. Morris and T. Sears, J. Chem. Sot. Faraday Trans. II 72 (1976) 597.
522
1 February
1977
[14] C.R. Bruadie. D. Neumann, WC. Price, D. Evans, A.W. Potts ant D.G. Streets, J. Chem. Phys. 53 (1970) 705. 115 ] D.W. Tu:-ner. C. Baker, A.D. Baker and C.R. Brundle, Molecuk r photoelectron spectroscopy (Wiley-Interscience, New York. 1970). 1161 0. Edqvist, E. Lindbolm, LX. Selin, H. SjBgrcn and L. Asbrink, Arkiv Fysik 40 (1970) 439. [I7] 0. Edqvist, E. Lindholm. L.E. Selin and L. Asbrink, Phys. Scripta 1 (1970) 25. [ 18) L. Karlsson, L. Mat&on. R. Jadrny, R.G. Albridge, S. Pinchas, T. Bergmark and K. Siegbahn, J. Chem. Phys. 62 (1975) 4745.