Electronic spectroscopy and nanocalorimetry of hydrated magnesium ions [Mg(H2O)n]+, n = 20–70: spontaneous formation of a hydrated electron?

The absorption spectra and photochemistry of [Mg(H2O)n]+, n = 20–70, resemble those of the hydrated electron (H2O)n–.


Calculation of the relative photodissociation cross sections
Photodissociation cross sections σ are extracted from measured ion intensities via eq. (S1). (S1) Here, 0 is the parent ion intensity and is the intensity of fragment ion i. The laser wavelength is represented by λ, the number of pulses by p and the pulse energy by E; h is Planck's constant, c is the speed of light and A represents the area covered by the laser beam at the position of the ICR cell.
Here, 2 is the partial cross section for the water evaporation channel (I) and is the partial cross section for the hydrogen dissociation channel (II). 2 is the sum of the intensities of all [Mg(H2O)n] + products created with a single photon and all other fragments created with two or more photons and is the sum of the intensities of all [MgOH(H2O)n] + products created with a single photon.
The setup for the laser irradiation is shown in Figure S1. The number of pulses p is controlled by an electronically controlled shutter. The pulse energies E are measured by a power meter and absorptions of the CaF2 prisms and the CaF2 cell window are accounted for in the calculations. The wavelength λ is measured with a wavemeter. Figure S2 shows the power spectra of the laser system in the relevant photon energy range recorded during the experiments. The area A of the laser beam at the ICR cell is derived from the laser system manufacturer's specification of the beam diameter at the laser exit (5 mm, FWHM at 450 nm), the beam divergence (< 5 mrad, full angle at FWHM level at 450 nm) and the distance to the cell (4.6 m) in the experimental setup.

Mass spectra
Typical mass spectra for the observed ions are shown in Figures S16 -S33 at selected photon energies, as well as without irradiation.                    S30 Figure S44: Fits to determine the centre of the fragment cross sections for the loss of 11, 12, 13, 15, 16 and 18 water molecules for [Mg(H2O)50] + . For the loss of 11 water molecules, the fit for the lower energy data was not considered for the evaluation. The same is true for the loss of 14 water molecules (not shown in the figure). For the loss of 13 water molecules, the fit of the data points between 2.5 -3.0 eV did not converge. For the loss of 15 water molecules, the fit of the data points around 2.8 eV did not converge. The fit of de date for the loss of 17 water molecules did also not converge (not shown in the figure). Blue dots: data points considered for fitting, black dots: not considered, red line: Gaussian fit, gray line: noise level.

Correction of discontinuities in the spectra
At the wavelength where the laser system switches from SF/SH (Sum Frequency / Second Harmonic) generated beam to signal beam and from signal beam to idler beam, discontinuities appear in the photodissociation cross sections. This is most likely due to a spatial deviation of the different beams.
To correct this, data points were recorded as close as possible on either side of the wavelength where beams change. The cross sections were adjusted to meet at the same level at this specific wavelength. This is accomplished by multiplying the intensities outside the signal beam by a constant factor. The results are shown in Figures S51 -S55.