Daniele Romanini

Ring-down cavity absorption spectroscopy of the very weak HCN overtone bands with six, seven, and eight stretching quanta

A nonstandard, high sensitivity, absorption detection technique has been applied to the investigation of the very weak fifth, sixth, and seventh overtones of HCN at 100 Torr and 296 K. The frequency range covered is from 17 500 to 23 000 cm−1. We report high resolution, absolute absorption spectra with a noise equivalent sensitivity as low as ∼2×10−9/cm (recently improved to 7×10−10/cm). Band origins, rotational constants, and band intensities are reported and compared with calculated values. The HCN overtone spectra in the present study are not affected by any kind of perturbation, despite the high excitation energy involved.

Diode laser cavity ring down spectroscopy

Abstract We recently demonstrated how in cativy ring down spectroscopy (CRDS) a CW single frequency dye laser may be conveniently employed in place of the pulsed laser of standard CRDS. Here we extend this result to external cavity tunable diode lasers. Compact spectroscopic devices with extreme sensitivity (2 × 10−10/cm) become a reality. To demonstrate the instrumental resolution we obtained high quality NO2 spectra in a supersonic slit jet, with a residual Doppler width of about 250 MHz.

The superposition principle and cavity ring‐down spectroscopy

Cavity ring‐down is becoming a widely used technique in gas phase spectroscopy. It holds promise for further important extensions, which will lead to even more frequent use. However, we have found widespread confusion in the literature about the nature of coherence effects, especially when the optical cavity constituting the ring‐down cell is excited with a short coherence length laser source. In this paper we use the superposition principle of optics to present a general and natural framework for describing the excitation of a ring‐down cavity regardless of the relative values of the cavity ring‐down time, the input pulse coherence time, or the dephasing time of absorption species inside the cavity. This analysis demonstrates that even in the impulsive limit the radiation inside a high finesse cavity can have frequency components only at the natural resonance frequencies of the cavity modes. As an immediate consequence, a sample absorption line can be detected only if it overlaps at least one of the cavi...

Jet-discharge cavity ring-down spectroscopy of ionized polycyclic aromatic hydrocarbons: progress in testing the PAH hypothesis for the diffuse interstellar band problem

Naphthalene cations (C10H+8) were produced in a slit jet coupled with an electronic discharge, and cavity ring down was used to obtain its absorption spectrum in the region 645-680 nm. Two of the strongest C10H+8 bands previously characterized by matrix isolation spectroscopy were found, both with a fractional blue shift of about 0.5%. This is the first gas-phase electronic absorption spectrum of an ionized polycyclic aromatic hydrocarbon (PAH). This work opens the way for a direct comparison of laboratory PAH spectra with the diffuse interstellar bands (DIB), the origin of which still constitutes an open problem in astrophysics.

Modelocked cavity--enhanced absorption spectroscopy.

We demonstrate the principle of cavity enhanced absorption with femtosecond modelocked lasers. The wide spectral coverage allowed by these sources makes this a promising high–sensitivity linear absorption technique. The uniformity of the modelocked frequency comb is the feature allowing effective injection of a high finesse cavity. The smooth and stable laser spectral profile guarantees a good background for the intracavity sample absorption spectrum, recorded by a spectrograph and a linear detector array. With a modelocked Ti:Sa laser and a cavity of finesse F ≃420 (F/π is the enhancement factor) we obtain a 4 nm section of a weak overtone band in 40 ms with 0.2cm-1resolution, and a detection limit of 2 × 10-7/cm/√Hz.

Single-frequency tunable Sb-based VCSELs emitting at 2.3 /spl mu/m

We present a comparison between two kinds of single-frequency Sb-based semiconductor VCSELs operating at 2.3 /spl mu/m in continuous-wave regime at room temperature. These lasers are studied in view of application to spectroscopy or trace gas detection. Both are based on a molecular beam epitaxy grown half-VCSEL. In the first configuration, a dielectric mirror is deposited on top to form a microcavity, while in the second a concave mirror is used to form an external cavity. The external cavity VCSEL exhibits 5-mW output power, a narrow linewidth (<<20 kHz), and 50-GHz continuous frequency tunability.

Effects of laser phase noise on the injection of a high-finesse cavity.

We study the response of a high-finesse optical cavity to a cw laser during the laser frequency passage through resonance. For a laser that is spectraly larger than the cavity resonance, laser-field phase fluctuations are converted into amplitude fluctuations, and cavity injection is intrinsically noisy. We develop a model based on Schawlow-Townes spontaneous-emission laser broadening and discuss in detail its effects on high-sensitivity spectroscopic techniques such as cavity-enhanced absorption or cavity ring-down spectroscopy. We present realistic simulations of cavity injection during a sweep through resonance and calculation of statistical quantities such as the average injection efficiency. Agreement with experimental observations is established.

Results from the OSQAR photon-regeneration experiment : No light shining through a wall

A new method to amplify the photon-axion conversions in a magnetic field is proposed using a buffer gas at a specific pressure in a photon-regeneration experiment. As a first result, new bounds for mass and coupling constant for laboratory experiments aiming to detect any hypothetical scalars and pseudoscalars, which can couple to photons were obtained, excluding with 95% confidence level, the recently withdrawn PVLAS result.

Cavity ring‐down overtone spectroscopy of HCN, H13CN and HC15N

This paper reports the results of our use of Cavity Ring Down Spectroscopy to extend the study of highly excited vibrational states in HCN. We extend our previous study of the H12C14N isotopomer, reporting on some weaker bands between 17 500 and 19 500 cm−1. We also report spectra of overtone and combination bands with six, seven, and eight quanta of stretching vibration in the isotopomers H12C15N and H13C14N in the interval from 17 500 to 23 000 cm−1. The observed spectroscopic constants and band intensities are compared with calculated values. All but one of the observed bands can be fit to within experimental accuracy (∼0.02 cm−1) to the standard distortable–rotor Hamiltonian. The one perturbed band has been successfully analyzed in terms of three anharmonically coupled levels. An anomalous line intensity distribution has been observed in the 1115 and 0116 Π←Σ bands, which we believe is produced by Coriolis coupling.

Cavity ringdown spectroscopy: broad band absolute absorption measurements

Abstract Here we report further results in our recent extension of cavity ring down spectroscopy (CRDS) to CW single frequency lasers. We previously pointed out the excellent reproducibility of our spectra, in particular the baseline measurements obtained from the empty cavity. The ability of accurately measuring the zero absorption baseline is essential when studying very broad or congested absorption spectra or even continua. We demonstrate the performance of our CW-CRDS setup by obtaining the absolute absorption spectrum of a weak and broad overtone transition in CHF 3 . We also discuss how the present results will apply to conventional pulsed-CRDS.