Effect of Freezing out Vibrational Modes on Gas-Phase Fluorescence Spectra of Small Ionic Dyes

Abstract

While action spectroscopy of cold molecular ions is a well-established technique to provide vibrationally resolved absorption features, fluorescence experiments are still challenging. Here we report the fluorescence spectra of pyronin-Y and resorufin ions at 100 K using a newly constructed setup. Spectra narrow upon cooling, and the emission maxima blueshift. Temperature effects are attributed to the population of vibrational excited levels in S 1 , and that frequencies are lower in S 1 than in S 0 . This picture is supported by calculated spectra based on a Franck−Condon model that not only predicts the observed change in maximum, but also assigns Franck−Condon active vibrations. In-plane vibrational modes that preserve the mirror plane present in both S 0 and S 1 of resorufin and pyronin Y account for most of the observed vibrational bands. Finally, at low temperatures, it is important to pick an excitation wavelength as far to the red as possible to not reheat the ions.

Laser-Induced Fluorescence (LIF) Spectroscopy of Trapped Molecular Ions in the Gas Phase

“…Recently, Nielsen et al carried out cold-ion fluorescence measurements with a cylindrical Paul trap-based apparatus named LUNA-2 (luminescence instrument in Aarhus-2). 71,72 The fluorescence emission spectra of pyronin cation (PY + ), resorufin anion (R À ), and rhodamine cations R575 + and R640 + were measured at a trap temperature around 100 K. They found that the fluorescence emission spectra of these molecular ions are blueshifted and become narrow at low temperatures, in agreement with earlier work. 73 The detection schemes adopted in the works presented so far fall in the single-pass detection schemes, that is, the exciting photons interact with the trapped ion cloud in a single pass, and the collected fluorescence is collimated, filtered, and sent to a fiber-coupled detector.…”

“…In addition to the ambient temperature studies on fluorescence, a few research groups have addressed the lowtemperature dependence of fluorescence characteristics of the gas-phase ions. [70][71][72][73][74] The molecular species of interest are cooled to low temperatures through a method referred to as the buffer-gas cooling. 30 In this process, an ideal gas such as helium is filled into the ion trap at relatively high pressure (of the order of a millibar), where the background pressure is typically below 1 × 10 À7 mbar.…”

Intrinsic fluorescence from firefly oxyluciferin monoanions isolated in vacuo

“…Hence vibrational frequencies are lower in S 1 than in S 0 , and transitions associated with Δ v = 0 will reduce in energy with the vibrational quantum number v . 27…”

“…Again, this was observed for rhodamines, oxazines, and smaller ionic dyes with the xanthene core structure. 25,27 It should be mentioned that the photon energies are all well below the vertical detachment energy (3.26 eV), 23 and electron photodetachment is not an open channel.…”

“…23 Similar temperature effects were earlier reported for other ionic fluorophores and discussed in detail. 26,27 Briefly, the redshift of the main band with temperature is due to smaller force constants in S 1 than in S 0 of low-frequency modes for which highquanta states are populated even at room temperature. Hence vibrational frequencies are lower in S 1 than in S 0 , and transitions associated with Dv = 0 will reduce in energy with the vibrational quantum number v. 27 The cold-ions experiment was repeated with lower excitation wavelengths at 533 nm (Fig.…”