Total photoabsorption cross section of molecular nitrogen near 83.4 nm

Abstract

The photoabsorption cross section of molecular nitrogen near 83.4 nm has been measured at room temperature. The cross sections at the wavelengths of the 2s2p 4 4p•, 4P3/: ' and 4Pin (all -2s22p34S%:) O* emission lines are measured to be 10.1 + 1.6 Mb, 0.29 _+ 0.02 Mb, and 0.049 _+ 0.008 Mb respectively. These measurements were made with an O'emission discharge source, and repeated using a synchrotron radiation source. Both measurements were made wifl'• the 0.008-nm resolution 6.65-m spectrometer at the SURF lI storage ring of the National Institute for Standards and Teclmology. INTRODUCTION The O + triplet near 83.4 nm is of great interest in ionospheric physics primm'ily for its role in remote sensing of the ionosphere. By compm'ing obse•wations of the 83.4-nm emission to the results of model calculations, the F2 region O + density and hence the electron density can be inferred (see, for example, Anderson and Meier [1985]). It has recently been pointed out [C!eaty et al., 1989] that absorption by molecular nitrogen strongly affects the intensity of this emission when observed at altitudes below 240 kin. In addition, Cleary et al. suggested that the overlap between the absorption lines of N2 and the emission lines of O* was much lower than previously indicated by Hudson and Carter[ 1969]. In this paper we report the results of a high-resolution laboratory measurement of the cross section of N2 in the vicinity of the O + 83.4-nm triplet. BACKGROUND The O + 83.4-nm emission lines are 2s 2p 4 4psa _ 2S 2 2p3 48o3a at 83.447 nm, 2s 2p 4 4P3a -2S 2 2p 3 '•Sø3,2 at 83,333 nm, and 2s 2p 4 '•P•/2 -2S 2 2p 3 •Sø3a at 83.276 nm. The source of this emission in the upper atmosphere is primarily photoionization-excitation of atomic oxygen. Below 240 km the density of molecular nitrogen is sufficiently high to cause significant absorption of the O + 83.4-nm emission. Carroll and Yoshino [1972] and Ledbetter [1972] have provided high-resolution spectrograms of the N2 absorption in this region. Although these photographic studies clearly show the complex rotational bands in the N2 spectrum, they cannot determine absolute cross section values. Previous photoelectric surveys of the N2 absorption cross section have also been made [Hudson and Carter, 1969; Carter, 1972; Gl'irtler et at., 1977]. Unfortunately, the agreement among these studies has been poor. For example, at 83.3332 nm G'drtler et al. show an N2 cross section of 19 Mb while Hudson and Carter measured a value of 3.2 Mb. According to our

Measurements of rovibronic line oscillator strengths of N2 in the 83.4 nm region: A high‐resolution high‐temperature study

“…Because of the presence of strong perturbations, overlapping, hot bands, and the extremely sharp N 2 absorption features a slight change in wavelength may result in a significant change of cross‐section value. We agree with Morgan et al [1993] that the uncertainty in the wavelengths of the OII multiplets and the wavelength calibration are the largest sources of error in the measurements. To compare with the previously reported measurements, we summarized the absorption cross‐section values in Table 3.…”

“…By examining the N 2 absorption cross sections shown in Figures 2a, 2b and 2c, we note that the base line absorption cross sections appear to slightly increase with temperature of the gaseous N 2 molecules from 295 K to 600 K. The increase can possibly be attributed to absorption from the ground electronic state to the continuum states of N 2 [ Morgan et al , 1993; Shaw et al , 1992; Wu et al , 1984] and hot band absorptions.…”

“…From Table 1 and according to , we sum up the integrated oscillator strength of each rovibronic line up to J = 16 for the (0,0) band of the c′ 6 1 Σ u + − X 1 Σ g + transition from 83.3277 nm to 83.4219 nm and obtain the integrated oscillator strengths of 8.34(±0.80) × 10 −4 , 8.93(±0.91) × 10 −4 , and 9.30(±1.07) × 10 −4 for temperatures of 295 K, 535 K, and 600 K, respectively. The increase of the integrated oscillator strengths as the temperature is increased can be due to contribution of absorption from the ground electronic state to the continua states of N 2 [ Morgan et al , 1993; Shaw et al , 1992; Wu et al , 1984] and from hot band absorptions and perturbations [ Wu et al , 1999; Wu et al , 1998]. The contribution from the overlap of the weak (2,0) band of the c 4 1 Π u − X 1 Σ g + and the (25,0) band of the b′ 1 Σ u + −X 1 Σ g + transitions on the long wavelength side, i.e., longer than the position of J = 16, becomes evident when we integrate the oscillator strengths from 83.3277 nm to 83.4770 nm.…”

“…To compare with the previously reported measurements, we summarized the absorption cross‐section values in Table 3. As mentioned by Morgan et al [1993], the N 2 absorption studies using a resolution of 0.003 nm or less [ Gurtler et al , 1977; Carter , 1972; Hudson and Carter , 1969] show little or no structure in the 83.4 nm region. Therefore their cross‐section data may not be as reliable as those obtained by using higher resolution.…”

“…The photoabsorption cross‐section measurements of N 2 in the 83.4 nm region have been reported for various resolutions [ Morgan et al , 1993; Gurtler et al , 1977; Carter , 1972; Hudson and Carter , 1969; Cook and Metzger , 1964; Huffman et al , 1963]. Recently, Chan et al [1993] reported absolute photoabsorption oscillator strengths of the nitrogen molecule from 11 eV to 200 eV utilizing high‐resolution dipole (e,e) spectroscopy with a resolution of 0.048 eV fwhm (full width at half maximum), which corresponds to a 0.27 nm fwhm resolution at 83.4 nm (14.9 eV).…”