Upper limits for stereoselective photodissociation of free amino acids in the vacuum ultraviolet region and at the $\mathrm{C}1s$ edge

We measured the total and partial ion yields of the two chiral amino acids alanine and serine in the gas phase both in the vacuum ultraviolet region and at the $\mathrm{C}\left(1s\right)$ edge using circularly polarized light. We did not detect any circular dichroism asymmetry larger than $1\times {10}^{-3}$. A similar measurement of fixed-in-space amino acids yielded an upper limit of $1\times {10}^{-2}$ for the stereoselective effect of circularly polarized light. The results obtained are relevant for quantitative models of stereoselective photodecomposition of amino acids that try to explain the homochirality of life.

Figure 1

The ion spectrometer is a three element Wiley-McLaren-type time-of-flight spectrometer. A pulsed electric field is applied to the pusher. $+200\mathrm{V}$, the extractor electrode is grounded. The gap between the pusher and extractor is $8\mathrm{mm}$. A rectangular, $8\mu \mathrm{s}$ duration, $20\mathrm{KHz}$ repetition rate pulse generator with a rise time of better than $20\mathrm{ns}$ was used. The detector consists of a segmented anode.

Figure 2

The CD results for the total ion yield of randomly oriented chiral molecules in the vuv region.

Figure 3

The mass spectrum of L-alanine at $h\nu =18.1\mathrm{eV}$ (curve). The lines correspond to the mass spectrum from the NIST database [15]. With the exception of a strong contribution at $18\mathrm{amu}$ $\left({\mathrm{H}}_2\mathrm{O}\right)$, our data is consistent with Ref. 15.

Figure 4

Partial ion yield at the $\mathrm{C}\left(1s\right)$ edge of L-alanine. The solid line is the measured partial ion yield signal, the dotted line is a theoretical calculation [24]. The left scale refers to the measured ion yields, while the right scale (Mb) refers to the theoretical data. The lower plot shows the difference in the cross sections for light of opposite helicities. The photon energy resolution was better than $100\mathrm{meV}$ full width at half maximum (FWHM) during this scan. The photon energies of the theoretical data were shifted by $-4.25\mathrm{eV}$ to match the experimental data. The arrow indicates the energy of the CD measurement.

Figure 5

Partial ion yield of D-serine at the $\mathrm{C}\left(1s\right)$ edge. The solid line is the measured partial ion yield signal, the dotted line is theory [24]. The left scale refers to the measured ion yields, while the right scale (Mb) refers to the theoretical data. The photon energy resolution was better than $100\mathrm{meV}$ FWHM during this scan. Theory values have been shifted $-5.60\mathrm{eV}$ in photon energy to match the experimental data. The arrow indicates the energy of the CD measurement. The chemical shifts of the different $\mathrm{C}1s$ edges are clearly resolved and contribute differently to the different fragment masses (not shown here).

Figure 6

The mass spectrum of L-alanine at $h\nu =287.1\mathrm{eV}$.

Figure 7

The mass spectrum of D-serine at $h\nu =306.0\mathrm{eV}$. This spectrum contains strong contributions of ${\mathrm{H}}_2\mathrm{O}$ coming from the oven, as the sample drying procedure was too short in this case.

Figure 8

A simple model for CD of amino acids attached to dust particles: If the molecules are attached with the same chemical group to a surface, e.g., the surface of dust particles, the molecules on the illuminated side have a preferred orientation with respect to the light. The electrical vector of the light counts the different chemical groups in the order ABCABCABC for the L form and in the order ACBACBACB for the D form. Thus CD is also possible in the dipole approximation.

Figure 9

With a position- and time-resolving ion detector, it is possible to select those events where a specific fragment of a specific mass is emitted in the forward direction. This incomplete measurement of the molecular orientation resembles the preferred orientation of molecules attached to spherical particles.