Experiments on formaldehyde scattering

rovibrational states produced in the collision the probe laser wavelength can be set to a suitable REMPI transition. During the experiment the wavelength can be monitored by a high-precision wavemeter (HighFinesse, WS7). The correct wavelength for a particular rovibronic transition belonging to the X-A-transition band system is calculated as described in Section 2.1. In some cases it is necessary to record an additional “background” trace in order to account for the scattering of states populated due to fluorescence out of the A state. In these cases the arrival time distribution is recorded twice: once with Dump laser blocked and once with Dump laser unblocked. The desired arrival time distribution assignable to incoming NO X²Π1

2(v = 11,J = 0.5) is obtained by subtracting the two traces from each other.

In these experiments the knowledge of the flight distances is critical. A method for their determination is described below. The x-position of a laser beam can be most accurately determined by the use of the surface manipulator. The laser beam pulse energy is monitored on the rear of the chamber while a micrometer screw is used for manipulating the x-position of the surface. When only half of the full laser beam pulse energy is transmitted through the chamber the surface clips half of the laser beam. Thus, in this configuration the micrometer screw will show the x-position of the center of the laser beam. The z-position is determined in two steps. First the ion signal corresponding to population of NO X²Π1

2(v = 11,J =0.5)in the incoming beam is recorded as a function of thez-position of the laser beam. The recorded maximum ion signal indicates the center of the molecular beam whose position can be read out at the micrometer screw. This position is then used as the origin of thez-axis and the laser beam can be moved to other z-positions, which can be easily determined by the reading of the micrometer screw on the periscope. In this way the distances l_i, l_ps, and l_s shown in Figure 4.1 are accessible from simple geometrical considerations.

Once d_ps, the arrival time of the incoming beam at x = l_ps, and the incoming beam velocity are known the arrival time of the incoming beam at the surface can be calculated.

4.2. Experiments on formaldehyde scattering

Molecular beams of formaldehyde are prepared by expanding the cracked monomer to-gether with different carrier gases. Since the nozzle reservoir is heated the molecule’s incidence translational energy can be adjusted up to 1.3 eV.

4. Experimental procedures

Probing scattered molecules

Preparation of vibrationally excited molecules

z

x

y Q

l

l

l

Preparation of vibrationally

excited molecules

z

x

y Probing incoming

molecules A)

B)

Figure 4.1.:Time-of-flight geometry used in the scattering of highly vibrationally excited nitric oxide. Panel A) shows the geometry for the experiment in the incoming beam. Panel B) shows the geometry for the experiment in the scattered beam.

48

4.2. Experiments on formaldehyde scattering

4.2.1. 1+1

REMPI of formaldehyde via the ˜ A state

The 1+1⁰REMPI of formaldehyde via the ˜A¹A2state[104]as a detection method has been established as a part of this thesis. Two laser pulses are used. The doubled output of the dye laser (Cobra-Stretch SL) at a wavelength around 353 nm (FWHM 8 ns) is used to excite molecules into the ˜A ¹A2 state. A VUV photon at 157 nm supplied by a fluorine laser (EX350 EXCIMER LASER, GAM Laser) is used for ionization of this state. The laser beams are collimated in the detection region. The timing of the laser pulses can be adjusted by means of delay generators (Stanford Research Systems, DG535). The delay between the laser pulses can most easily be measured by detecting the corresponding scattered laser light with the MCP detector. Irises can be used for both lasers.

The following settings are usually used in scattering experiments. The UV pulse energy is typically adjusted to ≈ 4 mJ/pulse. The beam diameter is reduced by a factor of two using a telescope to 3 mm and overlapped with the VUV laser which is delayed by 30 ns and focused as described in Section 3.2.4. At incidence translational energies below 0.4 eV the incidence beam and scattered molecules cannot be resolved by the ion time-of-flight. Thus scattered molecules are detected at x = 9 mm measured from the surface and z = 3 mm above the center of the incoming beam which excludes contributions of the incoming beam in the spectra of scattered molecules. Spectra of scattered molecules above 0.4 eV incidence translational energy can be acquired at a distance of 3 mm from the surface since incoming and scattered signal can be resolved in the ion time-of-flight.

See Section 3.1.2. The delay of the lasers with respect to the nozzle opening is set to the maximum of the arrival time distribution. The ion signal for scattered molecules is recorded as the dye laser is scanned with a scan speed of 0.002 nm/s.

4.2.2. 2+1 REMPI of formaldehyde via the 3 p

Rydberg state

2+1 REMPI of formaldehyde via the 3 p_x ¹A2 Rydberg state[25]is used in this work as a detection method for ground electronic state formaldehyde and is applied in non-state resolved time-of-flight experiments. The UV output of the Cobra-Stretch SL laser with doubling unit at around 295 nm (≈5 mJ/pulse) is focused using a lens with a focal length of 500 mm.

In most of the experiments the laser is set to 33866.6 cm⁻¹. According to a simulation with the spectroscopic constants taken from reference [105] the detected signal corresponds to transitions from many differentJ⁰⁰values but onlyK_a⁰⁰ = 0 andK_a⁰⁰ = 1. Thus, the REMPI signal should be approximately proportional to the population of the two K_a⁰⁰ manifolds

4. Experimental procedures

but should not be affected by population of higherK_a⁰⁰.

The arrival time distribution of the incoming or scattered beam can be measured as follows. The laser beam is moved to a certain set of x and z coordinates. The REMPI signal is recorded as the delay between laser firing and nozzle opening is scanned using a delay generator (Stanford Research Systems, DG535).

4.2.3. Preparation of formaldehyde in vibrationally excited states

The focused (f = 1000 mm) or unfocused output of the narrow-bandwidth IR-laser (≈ 4 mJ, beamdiameter≈ 4 mm) is overlapped with the molecular beam and collimated with the dye laser and F₂ laser used for 1+1⁰ REMPI spectroscopy as described in Sec-tion 4.2.1. The delay of the laser pulses is adjusted such that the IR pump pulse precedes the dye laser pulses by 10 ns. Excitation of 2141 or 2161 is probed via 1+1⁰ REMPI detection (see Section 4.2.1) of the vibrational ground and the vibrationally excited state.