Journal of Molecular Spectroscopy 215, 155–159 (2002) doi:10.1006/jmsp.2002.8625 High-Resolution Spectroscopy of Jet-Cooled Naphthalene: the 0 0 0 and 33 1 0 Bands of the ˜ A 1 B 1u ← ˜ X 1 A g Transition Duck-Lae Joo, ∗ Rika Takahashi, ∗ John O’Reilly, ∗ Hajime Kat ˆ o, ∗ and Masaaki Baba† ∗ Molecular Photoscience Research Centre, Kobe University, Kobe 657-8501, Japan; and †Faculty of Integrated Human Studies, Kyoto University, Kyoto 606-8501, Japan Received May 15, 2002 Rotationally resolved excitation spectra of the 0 0 0 and 33 1 0 bands of the ˜ A 1 B 1u ← ˜ X 1 A g electronic transition of naphthalene were measured by a frequency-doubled single-mode tunable laser and a jet-cooled collimated molecular beam. The observed linewidth was 18 MHz, and the absolute wavenumber was determined with an accuracy of better than 0.0002 cm −1 . The molecular constants of the ˜ X 1 A g (v = 0), ˜ A 1 B 1u (v = 0), and ˜ A 1 B 1u (v 33 = 1) levels were determined and represent the most accurate measurements to date. Three rotational constants were sufficient to fit 3386 lines of J = 1–43 and K a = 0–21 with a standard deviation 0.0002 cm −1 . This indicates that the molecular structures are rigid both in the ˜ X 1 A g and ˜ A 1 B 1u states. When a magnetic field was applied, spectral line broadening was observed for levels with small K a value in the ˜ A 1 B 1u (v 33 = 1) state, and the Zeeman splitting was observed to increase with increasing J . No broadening, however, was observed in the 0 0 0 band up to H = 0.65 T. C  2002 Elsevier Science (USA) 1. INTRODUCTION The molecular structure of naphthalene in the ground state was first determined in the solid phase via X-ray diffraction (1). Naphthalene is a planar, near-prolate molecule of D 2h symmetry with an asymmetry parameter of κ =−0.68. Departing from the selection of coordinate axes employed by Pariser (2), in this paper the top axis (a axis) is the z axis and the y axis is normal to the plane of the molecule (c axis). In this case, the nonzero matrix elements of the A-reduced rotational Hamiltonian H ( A) r in a basis of symmetric-top wave function | JKM  are given by (3)  JKM   H ( A) r   JKM  = 1 2 ( B + C ) J ( J + 1) +  A − 1 2 ( B + C )  K 2 ,  JK ± 2 M   H ( A) r   JKM  [1] = 1 4 ( B − C ){[ J ( J + 1) − K ( K ± 1)][ J ( J + 1) − ( K ± 1)( K ± 2)]} 1/2 , where A, B , and C are rotational constants. The conversion from Pariser’s convention x , y , z , a g , b 1g , b 2g , b 3g , a u , b 1u , b 2u , b 3u to the present one is z , x , y , a g , b 2g , b 3g , b 1g , a u , b 2u , b 3u , b 1u , respectively. There have been three principal no- tation systems for the normal vibrations (4–7 ). The vibration modes are numbered in the way described at the top of p. 272 of Herzberg’s book (8), and those used here are shown in Table 1. A band in the 312-nm absorption spectrum of naphthalene vapor was assigned as the electronic transition ˜ A 1 B 1u ← ˜ X 1 A g and was shown to consist of two main bands (9–12). A weak band polarized along the long axis (z ) with the vibrational band origin at 32 018.5 cm −1 was assigned as the 0 0 0 band. A strong band polarized along the short axis (x ), which gains intensity via vibronic coupling with the ˜ B 1 B 3u state (11), with the vibrational band origin at 32 453.5 cm −1 was assigned as the 33 1 0 band. The technique of crossing a laser beam at right angles to a jet- cooled molecular beam has been applied extensively (7, 13). In this technique, molecules undergo rotational and vibrational cooling, and the density of spectral lines is greatly reduced. This facilitates the assignment of transition lines. By the combination of a frequency-doubled, single-mode tun- able laser and a well-collimated supersonic molecular beam, Majewski and Meerts (14) observed the rotationally resolved spectra of the 0 0 0 and 33 1 0 vibronic bands of the ˜ A 1 B 1u ← ˜ X 1 A g electronic transition of naphthalene and naphthalene-d 8 with the linewidth 35 MHz. The rotational lines were assigned and the rotational temperature was 2.1 ± 0.3 K. However, the absolute transition wavenumber was determined by recording a Doppler- limited absorption spectrum of the iodine molecule and using this for calibration in comparison to an iodine atlas (15). The Doppler-free spectral atlas of the iodine molecules (16 ) is now available, and therefore the absolute transition wavenumbers can be calibrated with increased accuracy. Since the resolution is high, it is not necessary to lower the rotational temperature to 2 K, because the number of spectral line decreases, i.e., the informa- tion relating to higher J and K a levels is lost. J is the rotational quantum number and K a is its projection along the a-axis. In this paper, the most precise rotational constants recorded to date 155 0022-2852/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.