Appl. Phys. B 72, 245–248 (2001) / Digital Object Identifier (DOI) 10.1007/s003400000449 Applied Physics B Lasers and Optics Pressure broadening and shift of transitions of the first overtone of HCl M. De Rosa 1 , C. Nardini 1 , C. Piccolo 1 , C. Corsi 2 , F. D’Amato 3 1 LENS, European Laboratory for Non-Linear Spectroscopy, and INFM Largo E. Fermi 1, 50125 Firenze, Italy 2 Dipartimento di Scienze Neurologiche, Università di Firenze, Viale Morgagni 85, 50134 Firenze, Italy 3 S.I.T. srl, Via Masaccio 116, 50136 Firenze, Italy (Fax: +39-55/224-072, E-mail: derosa@lens.unifi.it) Received: 29 February 2000/Published online: 8 November 2000 –  Springer-Verlag 2000 Abstract. High-resolution spectroscopic measurements were made using distributed feedback diode lasers. We measured line strength and pressure-induced broadening and shift for two lines, R(3) and P (4), of the first overtone (2 ← 0) ro- vibrational band, for the two isotopomers H 35 Cl and H 37 Cl, according to their natural abundances; measurements were also made in the presence of foreign gases. Comparison was made with available data when possible. PACS: 33.20Ea; 33.20Vq; 33.70Jg Hydrogen chloride has assumed a relevant role in atmo- spheric chemistry, as it is the main reservoir of chlorine in the stratosphere and plays a fundamental role in the chem- istry of ozone. In the last few years several campaigns have been made in order to measure the stratospheric concentration of HCl [1]. HCl is also a product of many human activities, such as waste combustion in incinerators. Knowledge of the spectroscopic parameters such as line strength and pressure- induced broadening and shift is of great help in designing optical sensors for trace detection [2]. In addition they are useful for testing the intermolecular potentials and collisional models. In this article, we present high-resolution measurements of line strength and pressure-induced broadening and shift for two lines, R(3) and P (4), of the first overtone (2 ← 0) ro- vibrational band of the two isotopomers H 35 Cl and H 37 Cl; measurements were also made in the presence of foreign gases. A high-resolution profile of the absorption lines was obtained by using two different semiconductor diode lasers whose bandwidth is typically ∼ 10 MHz, smaller than the typical width of the absorption lines, e.g. for HCl at room temperature the Doppler width at λ = 1.8 μ m is 350 MHz. The diode laser can be continuously tuned over a frequency range of about 10–12 cm -1 by changing the temperature of operation. A finer sweep across the line can be made by vary- ing the injection current, from which a detailed profile of the absorption line is obtained. The rotational constant B of HCl is typically of the order of 10 cm -1 , and the spacing between two nearest lines is about 20 cm -1 , which is higher than the typical tunability range of our diode lasers. Thus, at most one single rotational line can be investigated with one diode laser. However, in our mixture of HCl two different isotopomers are present with their natural abundances: H 35 Cl and H 37 Cl with rela- tive abundances of 75.77% and 24.23% respectively [10]. The difference of the reduced mass for these isotopomers is very small, so the isotopic shift is of the order of a few cm -1 , and the spectra of the two species are very close. Their intensities are proportional to the abundance of the different isotopomers. As a result, in the range of emission of each of our diode lasers, two lines fall with the same rotational number but correspond to two different isotopic species. 1 Experimental setup Two different semiconductor diode lasers are used, both InGaAs/P distributed feedback (DFB) diode lasers: the first one (IMC) emits in the range around 1742 nm, where the R(3) lines for both the isotopomers occur, and it has a band- width of 10 MHz and a threshold current of 60 mA; the sec- ond DFB laser (sensor unlimited) emits around 1793 nm, where the two P (4) lines occur, and it has a bandwidth of 30 MHz and a threshold current of 30 mA. The lasers op- erate at room temperature (0–40 ◦ C) and the temperature is stabilized within 1 mK by means of a temperature con- troller. A coarse tuning is made by changing the tempera- ture, while a fine tuning is made by adjusting the injection current provided by a low noise current generator. A cur- rent ramp is added to the injection current in order to sweep the frequency periodically across the investigated transition line. The emitted radiation is collimated by a lens and split by means of a beam splitter, as sketched in Fig. 1. The reflected beam goes through a 25-cm-long confocal Fabry–Perot inter- ferometer, which marks the frequency scale. Its free spectral range has been measured to be 298 ± 2 MHz. For the meas- urement of the pressure-induced broadening and shift the transmitted beam is sent through two cells in cascade: a ref-