Internal-state thermometry by depletion spectroscopy in a cold guided beam of formaldehyde

We present measurements of the internal state distribution of electrostatically guided formaldehyde. Upon excitation with continuous tunable ultraviolet laser light the molecules dissociate, leading to a decrease in the molecular flux. The population of individual guided states is measured by addressing transitions originating from them. The measured populations of selected states show good agreement with theoretical calculations for different temperatures of the molecule source. The purity of the guided beam as deduced from the entropy of the guided sample using a source temperature of $150\mathrm{K}$ corresponds to that of a thermal ensemble with a temperature of about $30\mathrm{K}$.

Figure 1

(Color online) Experimental setup. Formaldehyde molecules from an effusive source are injected into the quadrupole guide and led to the QMS cross-beam ionization volume. In the last $15\mathrm{cm}$ long straight section the UV beam is overlapped with the molecular beam. The light is switched on and off by an electro-optical modulator placed between two polarizers.

Figure 2

(a) Time-resolved ion count rate measured with the QMS. The UV light is on between 0.5 and $1.1\mathrm{s}$. (b) The transition $\mid 3,3,0⟩\to \mid 4,4,1⟩$ measured at an electrode voltage of $\pm 5\mathrm{kV}$. The solid line is a fit to the measured line shape taking into account the laser beam and molecular beam profile and the electric field distribution in the spectroscopy section. The arrow indicates the laser frequency for measurements of the power dependence. (c) Laser power dependence of the depletion signal. The solid line shows the result of a fit using $\Delta \left(P\right)$. For (c) the electrode voltage was reduced to $\pm 1\mathrm{kV}$.

Figure 3

(Color online) Temperature dependence of population for selected guided states $\mid J,\tau ⟩$. (a) Calculated population of states for a guiding field of $20\mathrm{kV}∕\mathrm{cm}$. (b) Experimentally determined and calculated population relative to state $\mid 3,3⟩$ measured at $\pm 1\mathrm{kV}$ electrode voltage.