Mass detection and data processing

A voltage pulse from clusters impacting on the MCP detector is assigned to a spe-cific time-of-flight within a multiple event time detection system (FAST ComTec GmbH, P7887 multiscaler).^[90,216] As the voltage pulses have widths of few ns, 32 channels with widths of 250 ps were binned to 8 ns channels. The voltage amplitude detection threshold was adjusted such that the highest possible ion count rates were measured and the ratio of cluster signal and noise counts was not lowered due to increased noise generation.

In order to reliably determine the peak positions and integration borders of cluster peaks in the mass spectra, a summed mass spectrum was generated.^[203]In this spec-trum the peak positions of Na(H2O)n+ were determined as follows: After binning the ion counts in 200 ns segments, the first peak maximum was determined. For this purpose the peak of the Na(H2O)4+ cluster was used. The subsequent peak position of Na(H2O)5+ was found by the definition of a peak threshold level of 91%

of the peak height of the previous signal for the next 200 ns bin.^[203] As the cluster peaks are separated by few µs, this 9% lowering of the peak threshold level was applied several consecutive times resulting in a exponential decay with increasing peak separation (0.91^{(xbin−xbin,n=4)}). Further peak positions were determined via the same approach, but starting from the beforehand determined cluster peak. The aim of this approach was to neglect low-intensity mass peaks between the peaks of Na(H2O)n+ in the mass calibration procedure.^[203] These small clusters (see Figure 2.6 on page 33 at cluster sizes 30.5, 31.5, . . . ) are a product of multiple sodium doping in the pickup cell and belong to Na(NaOH)2(H2O)l+ clusters.^[152,153]

After the rough determination of cluster peak positions was completed, the peak maxima and the peak integration borders were re-evaluated by fitting Gaussian peak profiles around the ranges of the determined peak positions.^[203] The resulting mass calibration has been tested against a series of mixed phenol ethanol clusters ionized via REMPI with 275 nm photons, see Figure 3.4. As the time of flight is pro-portional to the square root of the mass-to-charge ratio (t_TOF ∝^qm/z), the peaks for large clusters (n > 100 −120) cannot be resolved properly and overlap until only a broad, log-normal shaped peak is observed.[34,90,127] The mass calibration for this region is based on the proper determination of the cluster masses in the range of separated signal peaks. Based on the relation tTOF ∝^qm/z the two calibration parameters c (a constant) and t₀ (the flight time of an ion with zero mass) can be

3.3 Mass detection and data processing

Figure 3.4: Peak position of Na(H2O)n+ and phenol ethanol clusters. The parameter t₀ was ob-tained from the mass calibration of the Na(H2O)n+ clusters via equation (3.4).

introduced^[93,139]

m_n

zn = m_n

1 = (t_n−t₀)²

c (3.1)

and calculated from the position of two cluster peaks with known masses m_n and charges z_n= 1 via

t₀ = m₁t₂−m₂t₁+√

m₁m₂ · |t₂−t₁|

m1−m2 (3.2)

and

c= (t1−t0)²

m₁ (3.3)

or from a quadratic regression from the peak positions^[203] via m

z = 1

ct²_TOF− 2t₀

c ·t_TOF+ t²₀

c (3.4)

If the two calibration parameters are known, the TOF can be transferred to a mass scale. The cluster size n, denoting the number of water molecules of the mixed

sodium water clusters, is then obtained via n= m_n−m_Na

m_H2O (3.5)

Here, m_Na refers to the atomic weight of sodium andm_H2O to the molecular weight of water. m_ndenotes the masses of cluster peaks obtained from the mass calibration of the time-of-flight mass spectra.

In order to measure two mass spectra simultaneously—a reference spectrum with only the UV laser used for ionization and a mass spectrum from IR-UV double reso-nance experiments—, a 10 ms TTL pulse with 5 Hz repetition rate (in the following TAG) was fed into the P7887 multiscaler to yield the assignment of the detected pulses to the correct mass spectrum.^[203,204] This TAG pulse was generated in a logic box whenever both Nd:YAG pump lasers, for the UV laser and the IR laser, were Q-switched. As the IR-OPO/OPA’s pump laser Q-switch was operated at halved frequency of the UV laser’s pump laser, spectra of photoionized clusters with and without IR excitation could be measured simultaneously. The delay time ∆t be-tween the laser pulses was adjustable in the range bebe-tween −50 ns and tens of µs and is defined as^[203]

∆t =t_UV−t_IR (3.6)

The laser pulse timest_UV and t_IR were determined as rising slopes of the laser pulse as detected by a photodiode (Electro-Optics Technology, Inc., Silicon PIN Detector ET-2000) and observed on an oscilloscope (Tektronix, TDS 744A).

4 Single photon ionization of Na(H ₂ O) _n clusters

In this chapter, the results from a single photon ionization study on sodium doped water clusters are presented. The photon energy dependent ion yield of small sodium water clusters is compared to the results from ab initio molecular dynamics simula-tions provided by the group of Prof. Petr Slavíček from the Department of Physical Chemistry of the University of Chemistry and Technology Prague. Most of the results of this chapter were published in a joint publication, reference [72].

4.1 Experimental method

All experiments were conducted with the apparatus described in Section 3. Pure water clusters were formed by skimmed, rare gas-seeded, supersonic expansions from a conical nozzle. Sodium doping was achieved in a pickup cell. The Na(H2O)n

clusters were ionized by UV light in the energy range of 2.7−5.4 eV from either the Continuum Panther Ex OPO or the Sirah Cobra Stretch dye laser. The measured mass spectra were corrected for the laser pulse energies and the number of laser shots.