Low and room temperature photoabsorption cross sections of NH3 in the UV region

Abstract Using synchrotron radiation as a continuum light source, we have measured the absolute absorption cross sections of NH3 with a spectral bandwidth (FWHM) of 0.5 A. The photoabsorption cross sections of NH3 have been measured from 1750 to 2250 A under temperature conditions of 295, 195, and 175 K. Significant temperature effects in the absorption threshold region which are mainly due to the presence of hot band absorption are observed. The cross section value at peaks and valleys for the vibrational progressions of the (0,0) to (4,0) bands vary between −80% and +40% as the temperature of NH3 changes from 295 to 175 K. In contrast to this, the changes of cross section values, Pc,T, are found to vary less than 20% for the (v′, 0) vibrational progressions with v′ ⩾ 5. The measured separations between the doublet features of the (0,0), (1,0), and (2,0) bands are found to decrease as the temperature of NH3 decreases. The shifts of peak positions of Pc,T with respect to the corresponding room temperature absorption peaks show a sudden change at v′ = 3 which appears to agree with the trend observed in the homogeneous line widths of the vibrational bands of NH3 ( Vaida et al., 1987 ; Ziegler, 1985 ; Ziegler, 1986 ). The unusual behavior of the line widths has been attributed to the A state potential surface which has a dissociation barrier.

[1]  K. Watanabe.  Photoionization and Total Absorption Cross Section of Gases. I. Ionization Potentials of Several Molecules. Cross Sections of NH3 and NO , 1954 .

[2]  Paul Harteck,et al.  Ultraviolet absorption coefficients of CO2, CO, O2, H2O, N2O, NH3, NO, SO2, and CH4 between 1850 and 4000 A , 1963 .

[3]  L. Ziegler.  Resonance rotational Raman scattering of symmetric tops: A probe of molecular photodissociation , 1986 .

[4]  L. Ziegler.  Rovibronic absorption analysis of the à ← X̃ transition of ammonia , 1985 .

[5]  島内 みどり,et al.  G. Herzberg: Molecular Spectra and Molecular Structure. III. Electronic Spectra and Electronic Structure of Polyatomic Molecules, D. Van Nostrand, Prinston 1966, 745頁, 16.5×24cm, 8,000円. , 1968 .

[6]  V. Vaida,et al.  Dissociation of NH3 to NH2+H , 1987 .

[7]  M. Suto,et al.  Photodissociation of NH3 at 106–200 nm , 1983 .

[8]  D. Judge,et al.  Photoabsorption and photoionization cross sections of NH3, PH3, H2S, C2H2, and C2H4 in the VUV region , 1991 .

[9]  M. Ashfold,et al.  State selective photodissociation dynamics of à state ammonia. II , 1988 .

[10]  P. Avouris,et al.  Electronic band‐shape calculations in ammonia , 1981 .

[11]  Hideo Okabe,et al.  Photochemistry of small molecules , 1978 .

[12]  V. Vaida,et al.  The ultraviolet absorption spectrum of the à 1A‘2←X̃ 1A1 transition of jet‐cooled ammonia , 1987 .

[13]  M. Lenzi,et al.  Photodissociation of NH3 in the Vacuum Ultraviolet , 1967 .

[14]  R. B. Cohen,et al.  Spectroscopy and dynamics of jet‐cooled hydrazines and ammonia. I. Single‐photon absorption and ionization spectra , 1992 .

[15]  D. Judge,et al.  Photoabsorption and direct dissociation cross sections of C2H2 in the 1530–1930 Å region: A temperature dependent study , 1989 .

[16]  W. Harshbarger.  Franck–Condon Factors for the Ã←X̃ Transition of Ammonia with Anharmonic Potential Functions for the Ground and Excited States , 1970 .