RESUMO
We implemented an optical frequency standard based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) at 1.39 µm. The emission frequency of an extended-cavity diode laser was actively stabilized against the center of the 4(4,1)â4(4,0) transition of the H(2)(18)O ν1+ν3 band, under optical saturation conditions. The nonlinear regime of laser-gas interaction was reached by using an optical cavity with a finesse of about 8700. By filling it with an 18O-enriched water sample at a pressure of a few Pa, the Lamb dip could be observed with a full width at half-maximum of about 2 MHz. Absolute frequency stabilization was obtained by locking the cavity resonance to the center of the sub-Doppler signal, which was provided by the NICE-OHMS technique under the dispersion regime of operation. An Allan deviation analysis demonstrated a relative frequency stability of â¼5×10(-13) for an integration time of 1 s. For longer integration times, the flicker frequency noise floor set the stability at the level of 4×10(-14).
RESUMO
A linear 16-pole ion trap-based experimental setup has been designed, implemented, and characterized to investigate the photophysics of biomolecules in the gas phase. Electrospray ionization is employed to generate the ions in the gas phase at atmospheric pressure. The voltage configuration on the ion funnel, the ion optic device in the first vacuum interface, is used to control the energy of the ions. A home-built quadrupole mass-filter is utilized for the mass-selection of the ions of interest. A 16-pole ion trap designed and built in-house is implemented for ion trapping. The instrument's versatility and capability are showcased by demonstrating the fragmentation patterns of protonated and deprotonated tryptophan, as well as describing the photodetachment decay of deprotonated indole.
RESUMO
This review focuses on the laser-induced fluorescence (LIF) spectroscopy of trapped gas-phase molecular ions, a developing field of research. Following a brief description of the theory and experimental approaches employed in general for fluorescence spectroscopy, the review summarizes the current state-of-the-art intrinsic fluorescence measurement techniques employed for gas-phase ions. Whereas the LIF spectroscopy of condensed matter systems is a well-developed area of research, the instrumentation used for such studies is not directly applicable to gas-phase ions. However, some measurement schemes employed in condensed-phase experiments could be highly beneficial for gas-phase investigations. We have included a brief discussion on some of these techniques as well. Quadrupole ion traps are commonly used for spatial confinement of ions in the ion-trap-based LIF. One of the main challenges involved in such experiments is the poor signal-to-noise ratio (SNR) arising due to weak gas-phase fluorescence emission, high background noise, and small solid angle for the fluorescence collection optics. The experimental approaches based on the integrated high-finesse optical cavities employed for the condensed-phase measurements provide a better (typically an order of magnitude more) SNR in the detected fluorescence than the single-pass detection schemes. Another key to improving the SNR is to exploit the maximum solid angle of light collection by choosing high numerical aperture (NA) collection optics. A combination of these two approaches integrated with ion traps could transmogrify this field, allowing one to study even weak fluorescence emission from gas-phase molecular ions. The review concludes by discussing the scope of the advances in the LIF instrumentation for detailed spectral characterization of fluorophores of weak gas-phase fluorescence emission, considering fluorescein as one example.