Infrared spectroscopy

In the infrared spectra of irradiated, phosphine-bearing ices, critical functional groups of phosphorus oxoacids were identified at 5 K and also in the residues that remained after annealing the exposed ices to 300 K (Supplementary Tables 1 and 2)25. These bands include stretching modes of phosphorus‒oxygen single and double bonds [ν(P‒O) (800–950 cm−1), ν(P=O) (1140–1300 cm−1)], the deformation mode of the OPOH functional group [δ(O=P‒OH) (1550–1710 cm−1)], and the P‒OH moiety [ν(O‒H) (2170 and 2700 cm−1). These findings suggest that functional groups linked to oxoacids of phosphorus are the results of an exposure of the ices at temperatures as low as 5 K. Experiments with 18O-labeled ice constituents (H 2 18O and C18O 2 ) match the isotopic shifts determined for redshifted ν(P‒O) and ν(P=O) modes by ~30 cm−126,27. Therefore, the infrared analysis reveals the existence of functional groups [P‒O, P=O, and O=P‒OH] present in phosphorus oxoacids; however, considering that fundamentals and hence the absorptions of functional of oxoacids, such as phosphonic and phosphoric acid, fall in the same range, infrared spectroscopy does not allow an identification of individual oxoacids nor their isomers. In other words, functional groups are not unique to specific phosphorus oxoacids and hence do not identify any particular molecule. Therefore, alternative analytical techniques are critical.

PI-ReTOF-MS

To probe discrete molecular species, we exploited photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) during the annealing of the irradiated ices. This method represents a unique approach to identify gas phase molecules isomer-selectively after photoionization based on their distinct ionization energies28. Here, the subliming products were photoionized in separate experiments with 10.86 and 9.93 eV photons to elucidate the nature of the oxoacid isomer(s) synthesized. Accounting for the adiabatic ionization energies (IE) of the oxoacids (H 3 PO x ; x = 1–4) (Table 1), the energy of the 10.86 eV photon is above the IE of each oxoacid; therefore, if formed and if subliming, all oxoacids can be ionized at this photoionization energy. The use of PI-ReTOF-MS necessitated isotopically labeled H 2 18O and C18O 2 reagents to distinguish between products formed at identical masses. Phosphinic acid and hypophosphorous acid (H 3 PO 2 ) along with diphosphine (P 2 H 4 ) have a molecular weight of 66 amu; on the other hand, 18O-substituted phosphinic acid and hypophosphorous acid (H 3 P18O 2 ) shifts the mass by 4 to 70 amu, thus discriminating these acids from diphosphine. At the photoionization energy of 10.86 eV, the temperature-programmed desorption (TPD) profiles indicate that H 3 P18O and H 3 P18O 2 oxoacids, which are associated with signal at m/z = 52 and 70, respectively, are formed in both PH 3 ‒H 2 18O and PH 3 ‒C18O 2 systems (Fig. 1). As the irradiation current increases from 100 nA via 1000–5000 nA, the ion counts at m/z = 52 (H 3 P18O+) decrease by a factor of about eight in the PH 3 ‒H 2 18O system; simultaneously, the signal at m/z = 70 (H 3 P18O 2 +), which is absent at 100 nA, arises. This finding may suggest that phosphinic/hypophosphorous acid (H 3 P18O 2 ) is formed from phosphine oxide (H 3 P18O) likely via reaction with atomic oxygen. The analysis of the PH 3 ‒C18O 2 systems supports this conclusion: phosphinic/hypophosphorous acid (H 3 P18O 2 ; 70 amu) is only detected at higher irradiation currents of 1000 and 5000 nA. On the other hand, PI-ReTOF-MS did not succeed in the detection of phosphorous/phosphonic acid (H 3 P18O 3 ; P(18OH) 3 /HP18O(18OH) 2 ) or phosphoric acid (H 3 P18O 4 ) due to their low volatility that limits their sublimation from the substrate.

Table 1 Calculated adiabatic ionization energies and relative energies of various phosphorus oxoacids Full size table

Fig. 1 PI-ReTOF-MS data showing the temperature-programmed desorption profiles for phosphorus oxoacids. Each column displays the profiles for m/z = 52 (H 3 P18O), m/z = 70 (H 3 P18O 2 ), m/z = 88 (H 3 P18O 3 ), and m/z = 106 (H 3 P18O 4 ). Top: Ices of phosphine (PH 3 )–carbon dioxide (C18O 2 ) at irradiation currents of a 100 nA, 9.93 eV photoionization energy, b 100 nA, 10.86 eV photoionization energy, c 1000 nA, 10.86 eV photoionization energy, and d 5000 nA, 10.86 eV photoionization energy. Bottom: Ices of phosphine (PH 3 )–water (H 2 18O) at irradiation currents of a 100 nA, 9.93 eV photoionization energy, b 100 nA, 10.86 eV photoionization energy, c 1000 nA, 9.93 eV photoionization energy, d 1000 nA, 10.86 eV photoionization energy, and e 5000 nA, 10.86 eV photoionization energy. The PI-ReTOF-MS data for the blank experiments are shown in Supplementary Fig. 2 Full size image

Considering the signals detected at m/z = 52 (H 3 P18O+) and 70 (H 3 P18O 2 +) in the 10.86 eV experiment, we were interested in untangling the nature of the structural isomer(s) formed. Since the ionization energies of the H 3 P18O‒H 2 P18OH and H 2 P(18O)(18OH)‒HP(18OH) 2 isomer pairs are separated by more than 1.5 eV (Table 1), a second set of experiments was carried out at a photoionization energy of 9.93 eV. This energy is below the ionization energies of the phosphine oxide (H 3 P18O) and phosphinic acid (H 2 P(18O)18OH) isomers, but above the ionization energies of the hydroxyphosphine (H 2 P18OH) and hypophosphorous acid (HP(18OH) 2 ) isomers. A close inspection of the TPD profiles of m/z = 52 (Fig. 1) reveals that the TPD profiles are nearly identical at 10.86 and 9.93 eV, suggesting that at least the thermodynamically less stable hydroxyphosphine isomer (H 2 P18OH) is formed; since the absolute photoionization cross-sections of both isomers are unknown, the presence of phosphine oxide cannot be proven. However, Withnall and Andrews explored in previous matrix isolation experiments the chemistry of phosphine‒molecular oxygen samples26,27 and revealed the formation of hydroxyphosphine (H 2 POH) with smaller contributions of the phosphine oxide isomer (H 3 PO). Finally, we compare the TPD profiles of m/z = 70 recorded at 10.86 and 9.93 eV. Upon lowering the photon energy to 9.93 eV, the signal at m/z = 70 vanishes; therefore, we can conclude that only the thermodynamically preferred phosphinic acid isomer (H 2 P(18O)18OH) is formed, but no hypophosphorous acid (HP(18OH) 2 ). The higher molecular weight of phosphinic acid (70 amu) compared to hydroxyphosphine (52 amu) is also associated with an increase of the sublimation temperature in the range of 260–300 K in contrast to 160–240 K.

TOF-SIMS

Having established the synthesis of at least two of the simplest phosphorus oxoacids (hydroxyphosphine and phosphinic acid) and possibly phosphine oxide by exploiting PI-ReTOF-MS, we searched for higher molecular-weight oxoacids in the residues of the annealed samples utilizing time-of-flight secondary ion mass spectrometry (TOF-SIMS). TOF-SIMS facilitates the sputtering of the solid residues and detects the ions in both positive and negative ion detection modes (Fig. 2). Since the sputtering might also fragment the phosphorus oxoacids, these fragmentation patterns have to be determined. This assists in an identification of well-defined mass-to-charge ratios unique to each of the oxoacids and also allows a quantification of the oxoacids formed. The results of the calibration of phosphonic acid (H 3 PO 3 ), phosphoric acid (H 3 PO 4 ), and pyrophosphoric acid (H 4 P 2 O 7 ) are compiled in Supplementary Tables 3 and 4. The negative ion spectra are very sensitive to probe the oxoacids via their deprotonated parent molecules. Here, pyrophosphoric (H 4 P 2 18O 7 ) and phosphonic acid (H 3 P18O 3 ) can be identified in all residues via their unique signals of HP 2 18O 6 −/ H 3 P 2 18O 7 − and H 2 P18O 3 −, respectively. While pyrophosphoric acid has a fragment of small intensity for H 2 P18O 4 −, the low quantity of H 4 P 2 18O 7 in our residues contributes a minor amount to the moderately intense H 2 P18O 4 − signal, which can be attributed to phosphoric acid (H 3 P18O 4 ). As a general trend, the yield of each of these oxoacids increases with the irradiation dose; significantly enhanced yields are seen in carbon dioxide bearing ices compared to water-rich ices especially at higher doses. Although phosphinic acid is subliming at 260–300 K as verified in the PI-ReTOF-MS analysis, the SIMS analysis revealed a strong peak for H 2 P18O 2 −; this ion is not observed as a fragment from any calibration compound, but can be formally linked to phosphinic acid (H 3 P18O 2 ). A close look at the PI-ReTOF-MS data (Fig. 1) indicates that the intensity of m/z = 70 (H 3 P18O 2 +) does not completely lead to zero at 300 K; therefore, a fraction of phosphinic acid (H 3 P18O 2 ) is likely to reside in the solid residue. Although holding a lower sensitivity, the positive ion spectra confirm the assignments derived from the negative ion mode. Pyrophosphoric acid (H 4 P 2 18O 7 ), phosphonic acid (H 3 P18O 3 ), and phosphoric acid (H 3 P18O 4 ) could be detected via their protonated counterparts, i.e., H 5 P 2 18O 7 +, H 4 P18O 3 +, and H 4 P18O 4 +.

Fig. 2 SIMS data from residues of irradiated phosphine-doped ices. The spectra were recorded in the negative (top) and positive ion mode (bottom) correlated with the formation of 18O-substituted oxoacids formed in irradiated phosphine (PH 3 )–water (H 2 18O) (right) and phosphine (PH 3 )–carbon dioxide (C18O 2 ) (left) ices Full size image

Gas chromatography

Finally, the phosphorus oxoacids in the residues were also extracted, derivatized as trimethylsilyl (TMS) esters (–OSi(CH 3 ) 3 ), and analyzed via two-dimensional gas chromatography time-of-flight mass spectrometry. A TOF-MS was exploited to record the retention times along with the mass spectra (Supplementary Table 5). This protocol led to the detection of three phosphorus oxoacids (Fig. 3): phosphoric acid (H 3 P18O 4 ), phosphonic acid (HP18O(18OH) 2 ), and phosphinic acid (H 2 P18O(18OH)). Phosphoric acid could be identified via the molecular ion of the tris(trimethylsilyl)ester (18OP(18Osi (CH 3 ) 3 ) 3 ) at m/z (M+) = 322 and its fragment originating from the loss of a methyl group at m/z (M+−15) = 30729. Phosphonic acid was detected via the molecular ion of the derivatized phosphorous acid tautomer in the form of its tris(trimethylsilyl)ester (P(18OSi(CH 3 ) 3 ) 3 ) at m/z (M+) = 304 and its fragment of the methyl group loss at m/z (M+−15) = 30430. Finally, phosphinic acid could be sampled via its hypophosphorous acid tautomer as its bis(trimethylsilyl)ester (HP(18OSi(CH 3 ) 3 ) 2 ) at m/z (M+) = 214 and also by its fragment of the methyl group elimination at m/z (M+−15) = 199. Derivatization as trimethylsilyl (TMS) esters shifts the tautomeric phosphonic‒phosphorous and phosphinic–hypophosphorous acid equilibrium to the phosphorous and hypophosphorous acid esters (Supplementary Fig. 1)31. Calibration experiments suggest that the TMS derivative of pyrophosphoric acid as detected in small quantities via SIMS was found to be thermally unstable and decomposed on the GC columns32. Consequently, with the exception of pyrophosphoric acid, the GC×GC-TOF-MS analysis correlates exceptionally well with the SIMS data that detected key phosphorus oxoacids in the residues of the irradiated phosphine-doped interstellar analog ices.

Fig. 3 Multidimensional gas chromatogram showing 18O-labeled phosphorus oxoacids extracted from the residues. The atomic mass units 214 (×100) and 304 (×150) were selected for this multidimensional chromatographic representation. Partial GC×GC chromatogram of the separation of the two minor phosphorus oxoacids is shown top left. The unassigned peaks result from the silylation agent (BSTFA: TMCS 1%) that was injected in excess compared to the blank analysis (Supplementary Fig. 3) to avoid sample loss Full size image

Discussion

Having identified four monophosphorus oxoacids [hydroxyphosphine (H 3 PO: PH 2 OH(−I)), phosphinic acid (H 3 PO 2 : H 2 P(O)OH(+I)), phosphonic acid (H 3 PO 3 : HP(O)(OH) 2 (+III)), phosphoric acid (phosphoric acid (H 3 PO 4 (+V))] along with pyrophosphoric acid (H 4 P 2 O 7 (+V)) with phosphorus in four distinct oxidation states ranging from –I to +V, we are discussing now possible formation pathways. For simplicity in the following discussion, the 18O label is dropped. It should be noted that although the FTIR analysis provided evidence on the emergence of functional groups associated with phosphorus oxoacids even at 5 K, the FTIR data were unable to identify individual oxoacids due to overlapping absorptions of the functional groups. Therefore, kinetic profiles linked to the formation of individual phosphorus oxoacids could not be provided. However, a few important conclusions can be drawn. First, based on the PI-ReTOF-MS data recorded at 100, 1000, and 5000 nA, the yields of PH 2 OH(−I) and H 2 P(O)OH(+I) depend on the irradiation current and hence dose. Recall that in the PH 3 ‒H 2 O system, H 2 P(O)OH(+I) is absent in the 100 nA experiment, but emerges at 1000 nA. This observation suggests a sequential formation of phosphorus oxoacids via stepwise reaction of oxygen atoms starting with phosphine (PH 3 ). Considering the water- and carbon dioxide-rich ices, the radiolysis of water and carbon dioxide can generate electronically excited oxygen atoms in strongly endoergic reactions5,33,34,35; water can also decompose via the formation of atomic hydrogen and hydroxyl radicals (OH) (Eq. 1–3). The required energy for the bond dissociation can be supplied by the energetic electrons.

$${\mathrm{CO}}_2\left( {X^1\Sigma _g^ + } \right) \to {\mathrm{CO}}\left( {X^1\Sigma ^ + } \right) + {\mathrm{O}}\left( {\,{}^1D} \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = 7.59\,{\mathrm{eV}}$$ (1)

$${\mathrm{H}}_2{\mathrm{O}}\left( {X^1{\mathrm{A}}_1} \right) \to {\mathrm{H}}\left( {\,{}^2{\mathrm{S}}} \right) + {\mathrm{OH}}\left( {X^2{\Pi}_{\mathrm{\Omega }}} \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = 4.83\,{\mathrm{eV}}$$ (2)

$${\mathrm{H}}_2{\mathrm{O}}\left( {X^1{\mathrm{A}}_1} \right) \rightarrow {\mathrm{H}}_2\left( {X^1\Sigma _g^ + } \right) + {\mathrm{O}}\left( {\,^1D} \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = 6.74\,{\mathrm{eV}}$$ (3)

Simultaneously, the energetic electrons can also lead to a phosphorus–hydrogen bond rupture in phosphine leading to phosphino (PH 2 ) radicals (Eq. 4)36.

$${\mathrm{PH}}_3\left( {X^1{\mathrm{A}}_1} \right) \to {\mathrm{PH}}_2\left( {X^2{\mathrm{B}}_1} \right) + {\mathrm{H}}\left( {\,^2{\mathrm{S}}} \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = 3.51\,{\mathrm{eV}}$$ (4)

In the water‒phosphine system, the formation of PH 2 OH may proceed via a barrier-less recombination of the phosphino (PH 2 ) radical with the hydroxyl (OH) radical (Eq. 5). Alternatively, electronically excited oxygen atoms can insert without barrier into the phosphorus–hydrogen bond leading to PH 2 OH as well (Eq. 6). It is important to note that electronically excited oxygen atoms can also add without barrier to the phosphorus atom of phosphine yielding phosphine oxide (H 3 PO) (Eq. 7), which can then undergo hydrogen migration to form PH 2 OH26,27,37. Schmidt et al. suggested that a barrier between 3.01 and 3.73 eV separated both isomers38; this energy can be supplied by the energetic electrons as well. Recall that the formation of phosphine oxide could not be proven or disproven in our study. We would like to stress that Stief et al. also probed the gas phase kinetics of ground-state atomic oxygen with phosphine over a temperature range of 208–423 K revealing that ground-state oxygen preferentially adds to the phosphorus atom39,40; unfortunately, neither products were identified, nor the role of intersystem crossing from the triplet to the singlet surface were probed.

$${\mathrm{PH}}_2\left( {X^2{\mathrm{B}}_1} \right) + {\mathrm{OH}}\left( {X^2{\Pi}_{\mathrm{\Omega }}} \right) \to {\mathrm{PH}}_2{\mathrm{OH}}\left( {X^1A\prime } \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = - 5.27{\mathrm{eV}}$$ (5)

$${\mathrm{PH}}_3\left( {X^1{\mathrm{A}}_1} \right) + {\mathrm{O}}\left( {\,^1D} \right) \to {\mathrm{PH}}_2{\mathrm{OH}}\left( {X^1A\prime } \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = - 8.27\,{\mathrm{eV}}$$ (6)

$${\mathrm{H}}_3{\mathrm{PO}}\left( {X^1{\mathrm{A}}_1} \right) \to {\mathrm{PH}}_2{\mathrm{OH}}\left( {X^1A\prime } \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = 0.03\,{\mathrm{eV}}$$ (7)

Once PH 2 OH is formed, the addition of another electronically excited oxygen atom to the phosphorus atom produces H 2 P(O)OH (Eq. 8). Successive insertions of electronically excited oxygen atoms in phosphorus–hydrogen bonds may lead via HP(O)(OH) 2 (Eq. 9) to H 3 PO 4 (Eq. 10). Essentially, in this reaction sequence, up to four oxygen atoms are required to oxidize phosphine to ultimately phosphoric acid via stepwise oxidation. The release of up to four oxygen atoms requires 27.0 eV and 30.4 eV to be generated from water and carbon dioxide, respectively. Therefore, thermal reactions cannot lead to the oxoacids at 5 K, but cosmic-ray-triggered non-equilibrium chemistry is required to supply the required oxygen atoms (and possibly the hydroxyl radicals) for the oxidation process.

$${\mathrm{PH}}_2{\mathrm{OH}}\left( {X^1{\mathrm{A}}\prime } \right) + {\mathrm{O}}\left( {\,^1{\mathrm{D}}} \right) \to\\ {\mathrm{H}}_2{\mathrm{P}}\left( {\mathrm{O}} \right){\mathrm{OH}}\left( {X^1{\mathrm{A}}\prime } \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = - 7.68{\mathrm{eV}}$$ (8)

$${\mathrm{H}}_2{\mathrm{P}}\left( {\mathrm{O}} \right){\mathrm{OH}}\left( {X^1{\mathrm{A}}\prime } \right) + {\mathrm{O}}\left( {\,^1{\mathrm{D}}} \right) \to\\ {\mathrm{H}}_2{\mathrm{P}}\left( {\mathrm{O}} \right)\left( {{\mathrm{OH}}} \right)_2\left( {X^1{\mathrm{A}}\prime } \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = - 7.81\,{\mathrm{eV}}$$ (9)

$${\mathrm{H}}_2{\mathrm{P}}\left( {\mathrm{O}} \right)\left( {{\mathrm{OH}}} \right)_2\left( {X^1{\mathrm{A}}\prime } \right) + {\mathrm{O}}\left( {\,^1{\mathrm{D}}} \right) \to\\ {\mathrm{H}}_3{\mathrm{PO}}_4\left( {X^1{\mathrm{A}}} \right){\mathrm{\Delta }}_{\mathrm{R}}{\mathrm{G}} = - 7.62\,{\mathrm{eV}}$$ (10)

In regard to the conversion yields, the infrared analysis indicates that 76 ± 8% of the phosphine reacted in the 10:1 CO 2 :PH 3 ice; this was determined by utilizing the comparative area of the ν 2 and ν 4 bands of phosphine. Similarly, the ν 2 , 2ν 2 + ν 3 , and ν 1 + ν 3 bands of carbon dioxide show that 28 ± 1% of CO 2 was destroyed after the irradiation. This equates to 8 ± 2 × 1016 molecules of phosphine and 3 ± 1 × 1017 molecules of carbon dioxide destroyed by irradiation. Since each irradiated carbon dioxide molecule liberates only one oxygen atom, the ratio of available reactive phosphorus-to-oxygen atoms is thus 1.0:3.4 ± 0.8. If all of the reacted phosphine became incorporated into one of the three simplest oxoacids, the 8 ± 2 × 1016 molecules produced suggest that 2.1 ± 0.8 × 10−4 molecules were formed per eV of irradiation. The formation of H 3 PO 4 is limited by the number of oxygen atoms generated, and at most 7 ± 2 × 1016 molecules of H 3 PO 4 could form, which corresponds to a generation of 1.7 ± 0.6 × 10-4 H 3 PO 4 molecules eV−1. The ν 2 and ν L bands of water were analyzed for the 10:1 H 2 O:PH 3 ice, and 62 ± 2% of phosphine was found to have reacted, while only 37 ± 5% of the water was destroyed. This is equivalent to 6 ± 2 × 1016 phosphine molecules and 8 ± 3 × 1017 water molecules. In this case, much more oxygen was available compared to phosphorus, with a 1:12 phosphorus-to-oxygen atom ratio, and thus phosphorus will always be the limiting atom in the formation of the oxoacids. Here, the upper yields of 6 ± 2 × 1016 molecules of phosphorus oxoacids are formed at 1.6 ± 0.6 × 10–4 molecules eV−1.

Using reagent standards, the quantity of phosphoric acid (H 3 PO 4 ) in the residues can be determined via GC×GC-TOF-MS and compared to the infrared spectra to establish a reaction yield. The CO 2 :PH 3 ices produced 5 nmol (3 × 1015 molecules) of phosphoric acid, while only 1 nmol (6 × 1014 molecules) was detected from the H 2 O:PH 3 ice, which results in 7 ± 3 × 10–6 molecules of H 3 PO 4 per eV in the CO 2 :PH 3 ice and 2 ± 1 × 10–6 H 3 PO 4 molecules eV−1 in the H 2 O:PH 3 ice. Thus, phosphoric acid represents 4% of the reacted phosphine in CO 2 :PH 3 , while a 1% yield was found in the H 2 O:PH 3 system. As phosphoric acid was the most abundant compound detected in the residues, this indicates that most of the phosphorus sublimed during the TPD, for example, as diphosphane as detected experimentally.

In conclusion, by exposing phosphine (PH 3 )-doped interstellar analog ices to ionization radiation and exploiting an array of complementary in situ and ex situ analytical tools, the present study offers compelling evidence on a facile formation of distinct oxoacids of phosphorus: phosphinic acid (H 3 PO 2 ) P(I), phosphonic acid (H 3 PO 3 ) P(III), phosphoric acid (H 3 PO 4 ) P(V), and pyrophosphoric acid (H 4 P 2 O 7 ) P(V). The formation of those oxoacids can be initiated via a radical–radical recombination between the hydroxy (OH) and phosphino (PH 2 ) radicals (Eq. 5) or through insertion of electronically excited atomic oxygen, released by unimolecular decomposition of water and carbon dioxide, with phosphine-forming phosphinic acid (PH 2 OH) (Eq. 6). Once phosphinic acid (PH 2 OH) forms, successive addition and insertion of electronically excited oxygen atom to the phosphorus atom produce phosphonic acid (H 3 PO 3 ) and phosphoric acid (H 3 PO 4 ) (reactions 8–10). Overall, up to four oxygen atoms are needed to oxidize phosphine to phosphoric acid; this requires up to about 30 eV. Hence, thermal reactions cannot form oxoacids at 5 K, but cosmic-ray-triggered non-equilibrium chemistry is required to supply the required oxygen atoms (and possibly the hydroxyl radicals) for the oxidation process. Please note that our studies were carried out in the condensed (ice) phase, but not under single collision conditions in the gas phase41. Therefore, it is not feasible neither in the present studies nor in any other laboratory to determine the efficiency of each elementary reaction (oxidation step) involved in the formation of the individual oxoacids. This would require pulse-probe experiments with femtosecond (few 10 fs pulses) electron pulses penetrating the ice sample. These experiments do not exist yet.

Whereas on Earth, phosphine is classified as highly toxic and only slightly soluble in water, the present work reveals interstellar phosphine as a critical precursor in the synthesis of highly water-soluble phosphorus oxoacids prevalent in contemporary biochemistry. Phosphoric acid (H 3 PO 4 ) in particular presents a soluble source of phosphorus in the phosphorus (V) oxidation state as found in RNA/DNA and ADP/ATP. The identification of pyrophosphoric acid (H 4 P 2 O 7 )—a formal condensation product of phosphoric acid—is significant since our investigations expose that polyphosphates such as diphosphates found in ADP can be formed in interstellar ices upon interaction with ionizing radiation mimicking typical life times of molecular clouds of a few 106 years. The unsuccessful detection of triphosphoric acid (H 5 P 3 O 10 )—a precursor to the phosphorus backbone of ATP—could indicate that the phosphorus chemistry in interstellar ices ceases with the synthesis of pyrophosphoric acid over the lifetime of a molecular cloud, thus defining the molecular complexity of phosphorus-bearing oxoacids formed in extraterrestrial environments.