THE DOPPLER-SPLITTING METHOD FOR THE GROUND VIBRATIONAL STATE

about 230 kHz away. This measurement has been taken at a pressure of A novel technique of high-resolution spectroscopy has recently been about 0.2 Pa, so the linewidths are almost Doppler-limited and there is no developed in our laboratory (1). We used a new version of the IR–mmwave hope of improving the resolution without resorting to Doppler-free methods. double-resonance method to study the £6 A 1 excited vibrational state of An example of the ‘‘resonant’’ case is shown in Fig. 1c, which displays CH3I and to measure the frequency of rotational and vibrational transitions the measurement of the absorption spectrum as recorded when the laser is at sub-Doppler resolution (2). In this letter we describe the first application chopped and tuned very close to the IR resonance of line Y. The frequency of the same approach to studying the ground vibrational state of a molecule. scale and the pressure are the same as in Fig. 1b: it is evident that the The basic idea is similar: double resonance with collinear propagation of linewidth is smaller than before and, in addition, line Z is almost canceled, pump and probe beams, which causes velocity selection and allows subbeing reduced to a weak residual background. Last, but not least, the signalDoppler resolution; however, in this case, the laser is used to decrease the to-noise ratio is very good as well. So this technique is a powerful tool for population of the ground state, instead of populating an almost empty exsingling out a specific line in a crowded spectrum and allows accurate cited state. measurements which are otherwise impossible, e.g., lineshape and broadenThe intensity of the pump laser is modulated by a mechanical chopper. ing studies. In the absence of the laser a certain fraction of the microwave power is absorbed by the sample, while when the laser is present, the absorption is smaller because part of the molecules are removed from the lower state of microwave transition. The transmission of the mmwave radiation through the gas sample is measured using a synchronous amplifier referenced to the pump modulation. By this approach it is possible to observe the rotational transition of interest at sub-Doppler resolution, with the additional bonus of isolating it from other interfering transitions not connected to the laser excitation. Two qualitatively different conditions can be found when the double resonance signals are recorded. In the first (‘‘resonant’’) case the pump laser is tuned at, or very close to the IR resonance so that only the molecules with a null velocity component along the laser beam are excited. In this way one obtains a linewidth narrower than the Doppler limit. In the second case the laser is appreciably detuned, say by an amount DnIR, from the exact center nIR of the rotovibrational transition, so a splitting of the rotational line is observed into two narrow components which correspond to counterand copropagating radiations (see Figs. 2, 3 of Ref. 2). This Doppler-induced splitting amounts to Dnmm A 2nmm DnIR/nIR and can easily be changed by careful tuning of the laser frequency. As discussed in Ref. (2) for the case of an excited vibrational state, this approach allows the determination of the frequency nIR with sub-Doppler accuracy. Here we prove that it is possible to use it also in the ground state. The experimental setup has been described in Refs. (1, 2). We use a cavity spectrometer operating in the frequency region near 150 GHz to observe the rotational spectrum of CH3I, and in Fig. 1 we summarize the principle of the experiment. In Fig. 1a we show what is observed in a traditional experiment, without using a laser. The power transmitted by the resonator is plotted versus the mmwave frequency: when the sample gas is FIG. 1. The mmwave spectrum of CH3I near 149.95 GHz. A traditional present three lines (labelled as X, Y, and Z) appear, superimposed on the frequency scan of the sample transmission is displayed in (a) and shows resonator mode, in a frequency interval about 1.5 MHz wide. The assignthree lines superimposed on the resonator mode; trace (b) is an expanded ment for the line labeled as X is (J, K, F) A (10, 4, 19/2) R (9, 4, 17/2), version of its central part, where line Y is visible only as a shoulder of the for Y is (10, 8, 25/2) R (9, 8, 23/2), and for Z is (10, 5, 17/2) R (9, 5, 15/ stronger line Z. The third spectrum (c) is recorded by the double resonance 2). As clearly shown in Fig. 1b (which is an expansion of Fig. 1a) line Y method, using a CO2 laser operating on the 10-P(8) line at a small detuning (DnIR) from the infrared resonance of the R8(9) line. is barely visible, being heavily overlapped by the stronger line Z, which is