8 tion is done at zero electric field and so cannot distinguish between the various M sublevels. For CaF, the ground state hyperfine structure is resolved and for all data we detect on the F ′′ = 5 hyperfine component. Of the 11 sublevels of this component, 3 correlate to M$_{N}$ = 0 at high field, while the remaining 8 are distributed equally amongst the other 4 M$_{N}$ states (as shown in Fig. 7). In this section, we discuss experiments on the Stark deceleration of $^{7}$LiH in the | 1 , 0 〉 state and of CaF in the | 4 , 0 〉 state, in order to demonstrate our main findings regarding nonadiabatic transitions. Figure 8 compares experimental results for the deceler- ation of LiH with the results of simulations. In this exper- iment the electrodes are switched between V$_{hi}$ = 9 . 5 kV and ground, and the decelerator is operated at a syn- chronous phase angle of 51 $^{◦}$. The beam entering the de- celerator has a mean speed of 437 m/s and a temperature of 1.5 K. The figure shows the LiH signal as a function of time, the zero of time being the moment when the ab- lation laser fires. The curves are normalized to the peak of the corresponding curves obtained when the voltage is applied constantly to the decelerator. The lower trace in the figure shows the experimental data, which was also discussed in [6]. The narrow peak that arrives at ap- proximately 2.5 ms corresponds to molecules decelerated from 420 m/s to 213 m/s, while the broad distribution centred near 1.7 ms is due to the molecules that are out- side the phase-space acceptance of the decelerator. The upper trace shows the result of a numerical simulation of the experiment where we assume that the molecules always follow the electric field adiabatically. The longi- tudinal velocity distribution used in the simulations is a Gaussian distribution with centre and width set equal to those measured in the experiments. The simulation does a good job of predicting the arrival time, width and amplitude of the decelerated bunch of molecules. The shape of the non-decelerated distribution is also repro- duced satisfactorily, but its amplitude is not. Instead, the simulation predicts that this amplitude should be approximately 2.5 times larger than observed in the ex- periment. The experimental setup for producing, decelerating and detecting LiH has been described before [6, 17]. We have not reported the deceleration of CaF previously, but the decelerator we use has the same structure as in [6] and the methods we use to produce and detect CaF are described in [18]. Both molecular species are produced by laser ablation of a target into a supersonically ex- panding gas jet. For the LiH experiments, the target is Li and the carrier gas is a mixture of H$_{2}$ ( ≈ 2%) and Kr. For the CaF experiments the target is Ca and the carrier gas is a mixture of SF$_{6}$ ( ≈ 2%) and Xe. The temperature and pressure in the solenoid valve are ap- proximately 300 K and 3–4 bar respectively. The valve is fired at 10 Hz and emits gas pulses with a duration of approximately 200 µ s into the source chamber which is maintained below 10 − $^{4}$mbar. The fundamental output of a Q-switched Nd:YAG laser, with a pulse duration of 10 ns and an energy of 25 mJ, is used to ablate the tar- get outside the nozzle of the valve. The resulting beam passes through a 2 mm diameter skimmer and into a sec- ond chamber which houses the decelerator and the detec- tor. In the LiH source, most of the molecules are formed in the ground state and so we excite the molecules to the N = 1 state upstream of the decelerator [6]. In the CaF source about 1% of all the molecules are in the state of interest, the | 4 , 0 〉 state, and we do nothing to increase this fraction. The Stark decelerator has 100 stages and has the struc- ture shown in Fig. 1(a), the dimensions being r$_{0}$ = 1 mm, R = 1 . 5 mm and L = 6 mm. For the LiH experiments the centre of the first stage lies 135 mm from the valve nozzle, while for the CaF experiments this distance was 126 mm. Four high voltage switches are used to switch the electrodes between two voltages, ± V$_{hi}$ and ± V$_{lo}$ . In Fig. 1(a) V$_{lo}$ = 0, but in the experiments we can choose a different value in order to increase the minimum electric field in the machine, allowing the probability of nonadi- abatic transitions to be controlled. Next, we modify the trajectory simulations to include the effects of nonadiabatic transitions in a simple way. The transition probability is first calculated as a func- tion of the longitudinal position in the decelerator us- ing the methods described in Sec. II, for the voltage, rise time and fall time used in the experiment. We record the position of every molecule every time the decelera- tor switches, and then look up the probability of each molecule remaining in the | 1 , 0 〉 state after each switch. Multiplying together all these probabilities we obtain the total probability for each molecule to reach the end of the decelerator without switching state. We then take the detection probability to be proportional to this survival probability. The middle trace in Fig. 8 shows the result of the modified simulation. As expected from the discus- sion above, we find that nonadiabatic transitions lead to loss of the non-decelerated part of the beam, but have no After passing through the decelerator the time-of-flight profile of the molecular pulse is recorded, with 10 µ s resolution, by cw laser induced fluorescence. The LiH molecules are detected 785 mm from the nozzle by ex- citing the A $^{1}$Σ$^{+}$( v ′ = 4) – X $^{1}$Σ$^{+}$( v ′′ = 0) R(1) tran- sition at 367.2 nm. The CaF molecules are detected 810 mm from the nozzle by exciting the A $^{2}$Π$_{1}$$_{/}$$_{2}$ ( v ′ = 0) – X $^{2}$Σ$^{+}$( v ′′ = 0) Q(4) transition at 606.3 nm. The fluores- cence is imaged onto the photocathode of a photomulti- plier tube operated in photon counting mode. The detec- A. Setup B. Results III. EXPERIMENT