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Hyperfine structure in the à state of PH2 arising from the fermi interaction of 31P
Volume 53. number 3
HYPERFINE
CHEMICAL
STRUCTURE
PHYSICS
LETTERS
1 February
1978
IN THE z STATE OF PH,
ARISING FROti THE FERMI; INTERACTION OF 31P R.F. CURL *, Y. ENDO, M. KAKIMOTO, S. SALT0 and E. HIROTA Institure for MoZecdar Science, Okazaki 444. Japan Received 1 November
1977
The photoluminescence spectrum of PHI excited by a cw dye laser shows resolved 31P hyperfme structure for the 000 level even in the presence of Doppler broadening. The observed splittings give a Fermi contact constant for the (O,O,O) x ‘Al state of PHI of 1.8 k 0.1 GHz.
Normally the resolution of nucIear hyperfiie structure in the electronic spectrum of a moiecule requires some special technique such as saturation spectroscopy or microwave optical double resonance because the hyperfme splittings (typically ==iOOMHz) are less than the Doppler width (typically =Z1000 MHz fwhm). The 31P hyperfme splitting of the 0, (O,O,O) x ?A1 level of PHz proves ro be an exception to this general rule primarily because of the rather large magnetic moment of 31P and because of the fact that PH, in the x state is a 0 radical. The resulting Fermi contact interaction constant may be interpreted in terms of the 3s character of the odd electron orbital.
diameter of =I 5 mm normally employed_ Usually the fluorescence appeared as a faint red streak extending 2-10 cm down the tube depending on the pressure, flow rate, and the immediate past history of the system. Although the fluorescence was only faintly visible to the eye, when observed with a red sensitive photomultiplier (Hamamatsu R666S) it was generally only about a factor of five weaker than the visually much brighter yellow fluorescence of NH, excited with Rh 6G. Typically ~5 X I@ counts/s were obtained with = f3 optics collecting light from ~5 mm of the fluorescence streak and using two Hoya O-58 edge filters (cutting off X < 5600 A) to block the scattered laser light. The laser wavelength was monitored with a Spex 14018 double monochromator (0.85 m) calibrated using a Hg lamp.
2. Experimental
3. Observations
PH, was prepared by allowing H atoms formed by a microwave discharge in Hz to flow over red phosphorous. The linear flow rate cannot be reasonably estimated because of the geometry of the apparatus but was probably of the order of IO3 cm/s with a pressure at the point of observation of =lOO mtorr. Fluorescence was excited with a Spectra Physics model 580 A cw dye laser using rhodamine 110 and operated in single mode. Typical laser powers were 50 mW with a beam
The O-O band of the x + f7 system of PH, was rotationally analyzed by Dixon et al. [l] _They report the Ooo + 1 1o line at 18259.49 cm-l (vat) with the two components J= l/2 + 312 and l/2 + l/2 which are expected to be split by 0.28 cm-l unresolved. When the laser excited photoluminescence spectruni of this frequency region is examined, a pattern of six lines extending over about 0.4 cm-l is observed as shown in fig. 1. There are no other lines within 1 cm-l on the high frequency side of the pattern and none within 2 cm-1 on the low frequency side. The frequencies of the lines of fig. 1 relative to the strongest line in rhe pattern
I_ Introduction
*
Visiting Professor from Chemistry Department. sity, Houston, Texas 7700 1, USA.
536
Rice Univer-
Volume 53, number 3
CHEMICAL
PHYSICS LETTERS
1 February
1978
Fig. 2. The same set of lines as in tit. 1 excited with a focussed beam. A single 800 run lens focussed the beam from a 1 cm diameter spot on the lens. Laser power 90 mW.
DEF
ABC
vFig. 1_ The observed laser excited photoluminescence spectrum of PHa near 182595 cm?. Beam diameter ~1.5 mm.
(line E) are tabulated in table 1. In obtaining these frequencies it was assumed that the laser mode spacing is 390 MHz. The behavior of the lines with respect to laser power saturation was examined by comparing the spectrum obtained with an unfocussed beam with that obtained when the beam was focussed from a diameter of 1 cm with an 800 mm lens. The focussed excitation spectrum is shown in fig. 2. Clearly lines B, C, D, and E are much more easily saturated than lines A and F suggesting the pattern into these two groups. For two lines of equal intensity the lower N line would be easier to saturate, because there are fewer M components to contribute Tab!e 1 Observed frequencies of PHa lines in the Oee- 1te region Line
Y (GHz)
Assignment
Predicted intensity
A
-13.0
B
-9.9
Ooe-l,o,J
l/2-1/2,FO-1
1
C
-8.1
Oeu-lre,J
112-112, F 1-l
2
Fl-0
1
D
-1.7
Oeu-lte,J
l/2-3/2, FO-1
2
Ooo-lto,J
l/2-312.F
5
E
0 a)
?
1-2 Fl-1
F
i-2.6
1
?
a) The other frequencies are listed relative to the strongest line E.
to the intensity requiring a larger transition for an individual M component.
moment
4_ Assignment The fine structure constants obtained from the LMR spectrum [2] predict (neglecting matrix elements offdiagonal in N) that the components of ?? I to J = l/3 and J = 3/2 will be separated by 8.1 f. 0.1 CHz with J = 3/2 lower in energy. Therefore, neglecting nuclear hyperfine structure, the Ooo + -1 1o transition is expected to consist of two lines separated by 8.1 + 0.1 GHz with the higher frequency component (l/2 +- 3/2) twice as strong as the lower frequency component (L/2 + l/2). This is precisely the pattern found for lines C and E, but also for lines I3 and D in fig. I_ We interpret the splittings between lines B and C and lines D and E as arising from slP hypertine structure in the excited state Ooo level for the following reasons (fH = 0 both for OOo and I to): (1) The relative intensities in the pattern is correct. The predicted intensities along with the hyperfme assignments are given in table 1 and this matches the observed intensity pattern. (2) The sinnilar saturation behavior of these four lines suggests that they belong together. (3) The B-C and D-E splittings are very nearly the same and both the magnitude and direction of the differences between the B-C and D-E splittings are consonant with the estimated values of the ground state 31P hyperfine coupling constants [2] of (O)l = 224 MHz and (cc)r = 287 MHz as can be seen in table 2. 537
Volume 53, number 3
CHEMICAL PHYSICS LETTERS
Table 2 Calculation of the Fermi constant from the observed splittings Av (E-D) = 1.72 f 0.04 GHz Au (C-B) = 1.83 + 0.1 GHz
sound state hyperfine structure a) IreJ=3/2 E(F=2) - E(F=I) = 0.11 GHz = cc l,rJJ= l/2 E(F=l) - E(F=O) = 0.12 GHz =e corrections of observed splittings b, (0); = Av(E-_D) f ;CY = 1.8 1+ 0.04 GHz (0); = Av(C-B) -$ = 1.79 f 0.1 GHz combining the two estimates (0); = 1.8 + 0.1 GHz a) Assumiug (0); = 224 MHz and (cc)~ = 287 MHZ [21b) Since the DoppIer width is mucir greater than these spIirtings an average of the two unresolved lines weighted by their expected intensities is taken.
NO other PH2 lines have been found with resolved hyperfme structUre_ The x state 31P hyperfine splittings are dominated by the Fermi interaction. For kvels with J =N + I[;!, the expected splittings are [(IV + l)/ (Uv + I)] (0), so that for N = 0, 1,2, ___ the coeffcients of (0)t are 1,2/3,3/S,. . ., l/2. Computation of the expected line shapes shows that resolution of the two components split by $ (O)* is not possible, because the Doppler width (fwhm) is 1.2 GHz. For N = 1 where the two components are split by 5 (0)1 computation predicts that a slight bulge on the side of the line on which the weaker component is located might be detectable under very careful examination_ Of course, unresolved 31P hyperfine structure should ma;rifest itself as line broadening particularly for N = 1, J = 312 and indeed the other PH2 lines are generally bl-oader. For levels with J =N - 112 the expected splittings are -[Nl(uv + l)] (0)1 so that for N = 1,2, . __ the coefficients OFT are -l/3, -2/S, ___, -l/2. Obviously
538
- 1 February 1978
from what has just been said transitions to these levels are not expected to have resolved hyperfme structure.
5. Discussion Using the ground state hyperfme structure corrections given in table 2, the Fermi constant obtained from the observed splittings is 1.8 a 0.1 GHz. The isotropic coupling constant expected [3] for 31P with the odd electron in a pure 3s orbital is 10.2 GHz_giving 18% 3s character for the odd electron in the A state of PH,. The observed valence angle [l] of 123” is consistent with sp2 hybridization which would lead to 33% 3s character for the odd electron on the P atoms since the odd electron orbital is located on P. As the valence angIe increases the s character of the odd electron orbital is expected to decrease to zero for the linear moiecule. It is iuterestingto compare the x 2A1 states of NH2 and PH,. The ‘;i: state of NH2 has equilibrium geometry which is nearly linear [4] _The Fermi constant for the II (0, 10,O) vibrational state of this level [S] corresponds to about 11% 2s character. The comparison is made difficult because the Fermi constant of the ll (0,10,O) state is an average over a wide amplitude of bound bending vibrationtbut it seems clear that the lower % 2s for NH, in the A state correlates with the larger bond angle. References [ 11 R.N. Di%on. G. Duxbmy and D-A_ Ramsay, Rot. Roy. Sot. A296 (1967) 136. [2] P.B. Davies. D.K. Russell and B-A_ Thrush, Chem. Phys. Letters 37 (1976) 43. [31 H.J. Bower, M.C.R. Symons and DJ.A. Tiding, in: Radical ions, eds. E-T. Kaiser and L. Kevan (Interscience, New York, 1968). [4] R.N. Dixon, Mol. Phys. 9 (1965) 357. [S] G-W. Hills, D.L. Philen, R-F. Curl and F.K. Tittel. Chem. Phys. 12 (1976) 107.
HYPERFINE
CHEMICAL
STRUCTURE
PHYSICS
LETTERS
1 February
1978
IN THE z STATE OF PH,
ARISING FROti THE FERMI; INTERACTION OF 31P R.F. CURL *, Y. ENDO, M. KAKIMOTO, S. SALT0 and E. HIROTA Institure for MoZecdar Science, Okazaki 444. Japan Received 1 November
1977
The photoluminescence spectrum of PHI excited by a cw dye laser shows resolved 31P hyperfme structure for the 000 level even in the presence of Doppler broadening. The observed splittings give a Fermi contact constant for the (O,O,O) x ‘Al state of PHI of 1.8 k 0.1 GHz.
Normally the resolution of nucIear hyperfiie structure in the electronic spectrum of a moiecule requires some special technique such as saturation spectroscopy or microwave optical double resonance because the hyperfme splittings (typically ==iOOMHz) are less than the Doppler width (typically =Z1000 MHz fwhm). The 31P hyperfme splitting of the 0, (O,O,O) x ?A1 level of PHz proves ro be an exception to this general rule primarily because of the rather large magnetic moment of 31P and because of the fact that PH, in the x state is a 0 radical. The resulting Fermi contact interaction constant may be interpreted in terms of the 3s character of the odd electron orbital.
diameter of =I 5 mm normally employed_ Usually the fluorescence appeared as a faint red streak extending 2-10 cm down the tube depending on the pressure, flow rate, and the immediate past history of the system. Although the fluorescence was only faintly visible to the eye, when observed with a red sensitive photomultiplier (Hamamatsu R666S) it was generally only about a factor of five weaker than the visually much brighter yellow fluorescence of NH, excited with Rh 6G. Typically ~5 X I@ counts/s were obtained with = f3 optics collecting light from ~5 mm of the fluorescence streak and using two Hoya O-58 edge filters (cutting off X < 5600 A) to block the scattered laser light. The laser wavelength was monitored with a Spex 14018 double monochromator (0.85 m) calibrated using a Hg lamp.
2. Experimental
3. Observations
PH, was prepared by allowing H atoms formed by a microwave discharge in Hz to flow over red phosphorous. The linear flow rate cannot be reasonably estimated because of the geometry of the apparatus but was probably of the order of IO3 cm/s with a pressure at the point of observation of =lOO mtorr. Fluorescence was excited with a Spectra Physics model 580 A cw dye laser using rhodamine 110 and operated in single mode. Typical laser powers were 50 mW with a beam
The O-O band of the x + f7 system of PH, was rotationally analyzed by Dixon et al. [l] _They report the Ooo + 1 1o line at 18259.49 cm-l (vat) with the two components J= l/2 + 312 and l/2 + l/2 which are expected to be split by 0.28 cm-l unresolved. When the laser excited photoluminescence spectruni of this frequency region is examined, a pattern of six lines extending over about 0.4 cm-l is observed as shown in fig. 1. There are no other lines within 1 cm-l on the high frequency side of the pattern and none within 2 cm-1 on the low frequency side. The frequencies of the lines of fig. 1 relative to the strongest line in rhe pattern
I_ Introduction
*
Visiting Professor from Chemistry Department. sity, Houston, Texas 7700 1, USA.
536
Rice Univer-
Volume 53, number 3
CHEMICAL
PHYSICS LETTERS
1 February
1978
Fig. 2. The same set of lines as in tit. 1 excited with a focussed beam. A single 800 run lens focussed the beam from a 1 cm diameter spot on the lens. Laser power 90 mW.
DEF
ABC
vFig. 1_ The observed laser excited photoluminescence spectrum of PHa near 182595 cm?. Beam diameter ~1.5 mm.
(line E) are tabulated in table 1. In obtaining these frequencies it was assumed that the laser mode spacing is 390 MHz. The behavior of the lines with respect to laser power saturation was examined by comparing the spectrum obtained with an unfocussed beam with that obtained when the beam was focussed from a diameter of 1 cm with an 800 mm lens. The focussed excitation spectrum is shown in fig. 2. Clearly lines B, C, D, and E are much more easily saturated than lines A and F suggesting the pattern into these two groups. For two lines of equal intensity the lower N line would be easier to saturate, because there are fewer M components to contribute Tab!e 1 Observed frequencies of PHa lines in the Oee- 1te region Line
Y (GHz)
Assignment
Predicted intensity
A
-13.0
B
-9.9
Ooe-l,o,J
l/2-1/2,FO-1
1
C
-8.1
Oeu-lre,J
112-112, F 1-l
2
Fl-0
1
D
-1.7
Oeu-lte,J
l/2-3/2, FO-1
2
Ooo-lto,J
l/2-312.F
5
E
0 a)
?
1-2 Fl-1
F
i-2.6
1
?
a) The other frequencies are listed relative to the strongest line E.
to the intensity requiring a larger transition for an individual M component.
moment
4_ Assignment The fine structure constants obtained from the LMR spectrum [2] predict (neglecting matrix elements offdiagonal in N) that the components of ?? I to J = l/3 and J = 3/2 will be separated by 8.1 f. 0.1 CHz with J = 3/2 lower in energy. Therefore, neglecting nuclear hyperfine structure, the Ooo + -1 1o transition is expected to consist of two lines separated by 8.1 + 0.1 GHz with the higher frequency component (l/2 +- 3/2) twice as strong as the lower frequency component (L/2 + l/2). This is precisely the pattern found for lines C and E, but also for lines I3 and D in fig. I_ We interpret the splittings between lines B and C and lines D and E as arising from slP hypertine structure in the excited state Ooo level for the following reasons (fH = 0 both for OOo and I to): (1) The relative intensities in the pattern is correct. The predicted intensities along with the hyperfme assignments are given in table 1 and this matches the observed intensity pattern. (2) The sinnilar saturation behavior of these four lines suggests that they belong together. (3) The B-C and D-E splittings are very nearly the same and both the magnitude and direction of the differences between the B-C and D-E splittings are consonant with the estimated values of the ground state 31P hyperfine coupling constants [2] of (O)l = 224 MHz and (cc)r = 287 MHz as can be seen in table 2. 537
Volume 53, number 3
CHEMICAL PHYSICS LETTERS
Table 2 Calculation of the Fermi constant from the observed splittings Av (E-D) = 1.72 f 0.04 GHz Au (C-B) = 1.83 + 0.1 GHz
sound state hyperfine structure a) IreJ=3/2 E(F=2) - E(F=I) = 0.11 GHz = cc l,rJJ= l/2 E(F=l) - E(F=O) = 0.12 GHz =e corrections of observed splittings b, (0); = Av(E-_D) f ;CY = 1.8 1+ 0.04 GHz (0); = Av(C-B) -$ = 1.79 f 0.1 GHz combining the two estimates (0); = 1.8 + 0.1 GHz a) Assumiug (0); = 224 MHz and (cc)~ = 287 MHZ [21b) Since the DoppIer width is mucir greater than these spIirtings an average of the two unresolved lines weighted by their expected intensities is taken.
NO other PH2 lines have been found with resolved hyperfme structUre_ The x state 31P hyperfine splittings are dominated by the Fermi interaction. For kvels with J =N + I[;!, the expected splittings are [(IV + l)/ (Uv + I)] (0), so that for N = 0, 1,2, ___ the coeffcients of (0)t are 1,2/3,3/S,. . ., l/2. Computation of the expected line shapes shows that resolution of the two components split by $ (O)* is not possible, because the Doppler width (fwhm) is 1.2 GHz. For N = 1 where the two components are split by 5 (0)1 computation predicts that a slight bulge on the side of the line on which the weaker component is located might be detectable under very careful examination_ Of course, unresolved 31P hyperfine structure should ma;rifest itself as line broadening particularly for N = 1, J = 312 and indeed the other PH2 lines are generally bl-oader. For levels with J =N - 112 the expected splittings are -[Nl(uv + l)] (0)1 so that for N = 1,2, . __ the coefficients OFT are -l/3, -2/S, ___, -l/2. Obviously
538
- 1 February 1978
from what has just been said transitions to these levels are not expected to have resolved hyperfme structure.
5. Discussion Using the ground state hyperfme structure corrections given in table 2, the Fermi constant obtained from the observed splittings is 1.8 a 0.1 GHz. The isotropic coupling constant expected [3] for 31P with the odd electron in a pure 3s orbital is 10.2 GHz_giving 18% 3s character for the odd electron in the A state of PH,. The observed valence angle [l] of 123” is consistent with sp2 hybridization which would lead to 33% 3s character for the odd electron on the P atoms since the odd electron orbital is located on P. As the valence angIe increases the s character of the odd electron orbital is expected to decrease to zero for the linear moiecule. It is iuterestingto compare the x 2A1 states of NH2 and PH,. The ‘;i: state of NH2 has equilibrium geometry which is nearly linear [4] _The Fermi constant for the II (0, 10,O) vibrational state of this level [S] corresponds to about 11% 2s character. The comparison is made difficult because the Fermi constant of the ll (0,10,O) state is an average over a wide amplitude of bound bending vibrationtbut it seems clear that the lower % 2s for NH, in the A state correlates with the larger bond angle. References [ 11 R.N. Di%on. G. Duxbmy and D-A_ Ramsay, Rot. Roy. Sot. A296 (1967) 136. [2] P.B. Davies. D.K. Russell and B-A_ Thrush, Chem. Phys. Letters 37 (1976) 43. [31 H.J. Bower, M.C.R. Symons and DJ.A. Tiding, in: Radical ions, eds. E-T. Kaiser and L. Kevan (Interscience, New York, 1968). [4] R.N. Dixon, Mol. Phys. 9 (1965) 357. [S] G-W. Hills, D.L. Philen, R-F. Curl and F.K. Tittel. Chem. Phys. 12 (1976) 107.