Hyperfine Structure in the A 3Π-X 3Σ- and d 1Π-a 1Δ Transitions of PF.
The system was first demonstrated by taking high resolution
spectra of the v'=2,3 and 5-7 bands of the A 3Π-X 3Σ-
(v',0) transition and the v'=4 - v"=0 band of the d 1Π-a
1Δ transition in PF. The spectra to the right show smaller and
smaller regions of the same spectrum; the first three were taken with a dye
laser and the last one with the high resolution OPO system. The structure
shown at each level is from a different physical origin. At the top is vibrational
structure, then spin orbit coupling, then rotational structure and finally
hyperfine structure. The moderate resolution dye laser spectra were taken
as a prelude to the high resolution study; the first ten members of the A-X
band system and the first five members of the d-a band system were recorded
this way. This provided improved rovibrational constants as compared to previous
work, and confirmed the unusual nature of the A state, with vibrational spacings
that increase for the first few vibrational levels. In addition, v'=4 of
the d state, probably the highest level that does not predissociate, was
observed for the first time.
Analysis
A selection of the bands were recorded at high resolution, and all showed
clear hyperfine structure. One rotational line from the A 3Π-X
3Σ- (6,0) band is shown to the right. Both the P and F
nuclei are spin ½, and interactions from both nuclei can be determined.
A band contour fit was used to determine hyperfine constants when the structure
was only partly resolved, as in the figure above. The excited
states are all essentially Hund's case (a) and the hyperfine structure is
given approximately by:
so for a 3Π1 component we are only determining two constants from the spectrum shown. For constants determined in this way each peak was recorded several times, and the error of the parameter derived from the reproducibility of the fits.
The hyperfine parameters themselves are determined by averages of quantities such as lzr-3 over the wavefunctions for the unpaired electrons where r is the distance from the nucleus in question. Comparing with average values of r-3 for atoms gives a means of interpreting these numbers. For PF we found:
| a1Δv=0 | A3Πv'=2 | A3Πv'=7 | d1Πv'=4 | |
| (3π) spin density (P) | 86% | 95% | 89% | 72% |
| (3π) spin density (F) | 10% | 12% | 12% | 9% |
| s character (P) | - | 3.0% | 4.6% | - |
| s character (F) | - | 1.5% | 1.0% | - |
The numbers are consistent with a pπ anti-bonding orbital mostly polarised towards phosphorus, and a pσ orbital with a small amount of s character from hybridisation. We covered a selection of vibrational levels in the A state but saw only slight changes in the hyperfine constants. The range covered does not correspond to a large change in the average bond length, so this is not unexpected. It would be interesting to investigate higher vibrational levels which would sample a wider range of internuclear distances, and this might be achievable with a double resonance technique.