TY - JOUR

T1 - Cryogenic fluorescence spectroscopy of oxazine ions isolated in vacuo

AU - Kjær, Christina

AU - Vogt, Emil

AU - Langeland, Jeppe

AU - Christensen, Nanna Falk

AU - Lindkvist, Thomas Toft

AU - Kjaergaard, Henrik G.

AU - Nielsen, Steen Brøndsted

PY - 2023

Y1 - 2023

N2 - Recent developments in fluorescence spectroscopy have made it possible to measure both absorption and dispersed fluorescence spectra of isolated molecular ions at liquid-nitrogen temperatures. Absorption is here obtained from fluorescence-excitation experiments and does not rely on ion dissociation. One large advantage of reduced temperature compared to room-temperature spectroscopy is that spectra are narrow, and they provide information on vibronic features that can better be assigned from theoretical simulations. We report on the intrinsic spectroscopic properties of oxazine dyes cooled to about 100 K. They include six cations (crystal violet, darrow red, oxazine-1, oxazine-4, oxazine-170 and nile blue) and one anion (resorufin). Experiments were done with a home-built setup (LUNA2) where ions are stored, mass-selected, cooled, and photoexcited in a cylindrical ion trap. We find that the Stokes shifts are small (14–50 cm−1), which is ascribed to rigid geometries, that is, there are only small geometrical changes between the electronic ground and excited states. However, both the absorption and the emission spectra of darrow-red cations are broader than those of the other ionic dyes, which is likely associated with a less symmetric electronic structure and more non-zero Franck–Condon factors for the vibrational progressions. In the case of resorufin, the smallest ion under study, vibrational features are assigned based on calculated spectra.

AB - Recent developments in fluorescence spectroscopy have made it possible to measure both absorption and dispersed fluorescence spectra of isolated molecular ions at liquid-nitrogen temperatures. Absorption is here obtained from fluorescence-excitation experiments and does not rely on ion dissociation. One large advantage of reduced temperature compared to room-temperature spectroscopy is that spectra are narrow, and they provide information on vibronic features that can better be assigned from theoretical simulations. We report on the intrinsic spectroscopic properties of oxazine dyes cooled to about 100 K. They include six cations (crystal violet, darrow red, oxazine-1, oxazine-4, oxazine-170 and nile blue) and one anion (resorufin). Experiments were done with a home-built setup (LUNA2) where ions are stored, mass-selected, cooled, and photoexcited in a cylindrical ion trap. We find that the Stokes shifts are small (14–50 cm−1), which is ascribed to rigid geometries, that is, there are only small geometrical changes between the electronic ground and excited states. However, both the absorption and the emission spectra of darrow-red cations are broader than those of the other ionic dyes, which is likely associated with a less symmetric electronic structure and more non-zero Franck–Condon factors for the vibrational progressions. In the case of resorufin, the smallest ion under study, vibrational features are assigned based on calculated spectra.

U2 - 10.1039/D3CP04615J

DO - 10.1039/D3CP04615J

M3 - Journal article

VL - 25

SP - 32715

EP - 32722

JO - Physical Chemistry Chemical Physics

JF - Physical Chemistry Chemical Physics

SN - 1463-9076

IS - 47

ER -