Stimulated-emission pumping (SEP) spectra of HCN have been measured by using a pulsed, tunable argon fluoride laser with a frequency-doubled, pulsed dye laser. Sixty-seven vibrational states of the ground electronic state between 8 900 and 18 900 cm−1 have been observed. Eighty percent of the states can be described within a traditional normal mode context. A full set of anharmonic vibrational constants was derived, unifying the SEP data reported here with previous infrared and overtone data. This set of molecular constants is expected to be able to predict the position of normal mode states below 19 000 cm−1 with an accuracy within 3 cm−1. Twenty percent of the states could not be assigned to unperturbed normal mode states, and a systematic analysis was performed in an attempt to find a simple explanation for them based on possible perturbations. Except for the lowest energies, no simple explanation was found, suggesting that delocalized isomerizing vibrational states are playing a role in the observed vibrational structure at higher energy. Direct comparison with assigned normal mode states derived from quantum-mechanical vibrational-structure calculations on the only available three-dimensional potential energy surface were made possible by these experiments. The deviation between experiment and theory as a function of the number of bending quanta, the vibrational motion that couples strongly to the isomerization reaction coordinate, makes clear that the isomerization barrier height is too low on this surface. The present state of experimental characterization of the HCN system should be good enough to permit a high-quality potential energy surface to be derived for highly vibrationally excited HCN.
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No information on the centrifugal distortion D constants could be obtained from these spectra. 53 218.3 cm−1 was used as the term energy of the upper state. Other constants used for the upper electronic state were B and D constants for the subscript d branches: 1.2649 and 7.4 × 10−6 cm−1 respectively. B and D for the subscript c branches were 1.3111 and 5.2 × 10−6 cm−1, respectively. All these constants were taken from Ref. 10.
Table 4
Observed and Calculated SEP Transition Vacuum Frequencies (in cm−1) for (0, 140,2, 3)
Calibration with I2 is not possible. These G values may be subject to as large as 2.0 cm−1 systematic error. The B constants should not be affected.
< ±0.001 cm−1.
< ±0.002 cm−1.
< ±0.003 cm−1.
< ±0.011 cm−1.
< ±0.009 cm−1.
< ±0.006 cm−1.
Table 8
Normal Mode Model’s Fit to the Observed Vibrational Term Values (in cm−1)a
Bold-faced lettering indicates the SEP data obtained in this work, while roman lettering is from the IR data of Ref. 27. Standard deviation of the fit, 0.811 cm−1.
Table 11
Full Set of Anharmonic Vibrational Molecular Constants (in cm−1) Resulting from Global Fit
Bold-faced lettering indicates bright state.
Prediction of global constants; estimated accuracy < 3 cm−1.
Prediction of Ref. 27.
I, May be consistent with Coriolis interaction (see text). II, Bright apparently unperturbed state observed and assigned. III, May be consistent with high-order Fermi and/or Coriolis interaction (see text). IV, No bright state available for this group. V, Search function would have to be in error bv 15–20 cm−1 (see text).
Tables (13)
Table 1
Spectral Transitions of HCN (in cm−1) Observed with a Tunable Argon Fluoride Laser: (0, 1, 0)1 ← (0, 20, 0)
Branch
J
Obs.
Calc.
Pd(J)
3
51 797.43
51 797.34
Pd(J)
4
51 793.35
51 793.33
Pd(J)
6
51 784.24
51 784.27
Pd(J)
7
51 779.27
51 779.22
Pd(J)
8
51 773.60
51 773.82
Pd(J)
9
51 767.88
51 768.07
Pd(J)
10
51 761.80
51 761.97
Pd(J)
11
51 755.43
51 755.53
Pd(J)
12
51 748.62
51 748.73
Pd(J)
13
51 741.34
51 741.58
Pd(J)
14
51 733.90
51 734.09
Pd(J)
15
51 725.94
51 726.24
Pd(J)
16
51 718.35
51 718.04
Qc(J)
2
51 805.88
51 805.96
Qc(J)
3
51 804.79
51 804.66
Qc(J)
4
51 802.91
51 802.90
Qc(J)
5
51 800.66
51 800.70
Qc(J)
6
51 798.08
51 798.06
Qc(J)
7
51 794.85
51 794.97
Qc(J)
8
51 791.55
51 791.45
Qc(J)
9
51 787.53
51 787.50
Qc(J)
10
51 783.06
51 783.06
Qc(J)
11
51 778.10
51 778.21
Qc(J)
12
51 772.68
51 772.90
Qc(J)
13
51 766.94
51 767.14
Qc(J)
14
51 760.98
51 760.94
Qc(J)
15
51 754.32
51 754.29
Qc(J)
16
51 747.31
51 747.18
Qc(J)
17
51 739.62
51 739.62
Qc(J)
18
51 731.84
51 731.60
Qc(J)
19
51 723.18
51 723.12
Qc(J)
20
51 714.27
51 714.19
Rd(J)
0
51 810.07
51 809.92
Rd(J)
1
51 812.17
51 812.19
Rd(J)
3
51 815.86
51 815.70
Rd(J)
4
51 816.76
51 816.93
Rd(J)
5
51 817.97
51 817.81
Rd(J)
6
51 818.54
51 818.35
Rd(J)
7
51 818.54
51 818.54
Rd(J)
8
51 818.54
51 818.37
Rd(J)
9
51 817.97
51 817.85
Rd(J)
10
51 816.76
51 817.00
Rd(J)
11
51 815.86
51 815.77
Rd(J)
12
51 814.32
51 814.20
Rd(J)
13
51 812.17
51 812.30
Rd(J)
14
51 810.07
51 810.00
Table 2
Spectral Transitions of HCN (in cm−1) Observed with a Tunable Argon Fluoride Laser: (0, 1, 0)1 ← (0, 22, 0)
J
Branch
Obs.
Calc.
Branch
Obs.
Calc.
3
Pc(J)
51 782.02
51 781.96
Pd(J)
51 782.25
51 782.24
4
Pc(J)
51 777.53
51 777.67
–
–
–
6
Pc(J)
51 767.88
51 767.74
Pd(J)
51 768.85
51 769.13
7
Pc(J)
51 762.21
51 762.12
Pd(J)
51 764.16
51 764.06
8
Pc(J)
51 756.06
51 756.05
Pd(J)
51 758.64
51 758.64
9
Pc(J)
51 749.65
51 749.53
Pd(J)
51 752.89
51 752.87
10
Pc(J)
51 742.59
51 742.57
Pd(J)
51 746.78
51 746.75
11
Pc(J)
51 735.21
51 735.17
Pd(J)
51 740.16
51 740.28
12
Pc(J)
51 727.29
51 727.32
Pd(J)
51 733.29
51 733.45
13
Pc(J)
51 718.87
51 719.01
–
–
–
14
–
–
–
Pd(J)
51 718.87
51 718.74
5
Qc(J)
51 800.66
51 800.70
Qd(J)
51 787.02
51 786.96
6
Qc(J)
51 798.08
51 798.06
Qd(J)
51 784.82
51 784.86
7
Qc(J)
51 794.85
51 794.97
Qd(J)
51 782.25
51 782.41
8
Qc(J)
51 791.55
51 791.45
Qd(J)
51 779.59
51 779.61
9
Qc(J)
51 787.53
51 787.50
Qd(J)
51 776.32
51 776.46
11
Qc(J)
51 778.10
51 778.21
Qd(J)
51 768.85
51 769.09
12
Qc(J)
51 772.68
51 772.90
Qd(J)
51 764.99
51 764.88
13
Qc(J)
51 766.94
51 767.14
Qd(J)
51 760.47
51 760.32
14
Qc(J)
51 760.98
51 760.94
15
Qc(J)
51 754.32
51 754.29
16
Qc(J)
51 747.31
51 747.18
Qd(J)
51 744.39
51 744.49
8
Rc(J)
51 799.05
51 799.02
10
Rc(J)
51 795.71
51 795.63
2
Rd(J)
51 799.05
51 799.02
4
Rd(J)
51 801.88
51 801.82
5
Rd(J)
51 801.88
51 801.77
Table 3
Molecular Constants for (0, 20,2, 0) and Comparison with Previous Results
No information on the centrifugal distortion D constants could be obtained from these spectra. 53 218.3 cm−1 was used as the term energy of the upper state. Other constants used for the upper electronic state were B and D constants for the subscript d branches: 1.2649 and 7.4 × 10−6 cm−1 respectively. B and D for the subscript c branches were 1.3111 and 5.2 × 10−6 cm−1, respectively. All these constants were taken from Ref. 10.
Table 4
Observed and Calculated SEP Transition Vacuum Frequencies (in cm−1) for (0, 140,2, 3)
Calibration with I2 is not possible. These G values may be subject to as large as 2.0 cm−1 systematic error. The B constants should not be affected.
< ±0.001 cm−1.
< ±0.002 cm−1.
< ±0.003 cm−1.
< ±0.011 cm−1.
< ±0.009 cm−1.
< ±0.006 cm−1.
Table 8
Normal Mode Model’s Fit to the Observed Vibrational Term Values (in cm−1)a
Bold-faced lettering indicates the SEP data obtained in this work, while roman lettering is from the IR data of Ref. 27. Standard deviation of the fit, 0.811 cm−1.
Table 11
Full Set of Anharmonic Vibrational Molecular Constants (in cm−1) Resulting from Global Fit
Bold-faced lettering indicates bright state.
Prediction of global constants; estimated accuracy < 3 cm−1.
Prediction of Ref. 27.
I, May be consistent with Coriolis interaction (see text). II, Bright apparently unperturbed state observed and assigned. III, May be consistent with high-order Fermi and/or Coriolis interaction (see text). IV, No bright state available for this group. V, Search function would have to be in error bv 15–20 cm−1 (see text).