Photoelectron imaging of PtI₂⁻ and its PtI⁻ photodissociation product

Platinum halides have complex electronic structures and exhibit unusual bonding, resulting in very high electron affinities (EAs) and high formal oxidation states on the platinum core. These properties have led platinum halides to be investigated as superhalogens and small multiply charged anions. For example, [PtCl₄]²⁻ is the smallest experimentally reported dianion¹ and PtF₆ has an EA of ∼7 eV and oxidizes both O₂ and Xe.^2–4 To date, the focus has been on platinum fluorides, chlorides, and—to a lesser extent—bromides, with no gas-phase studies investigating the electronic structure of the platinum iodides.^5–7 Hence, there are unanswered questions about how the interesting properties that characterize the smaller platinum halides evolve for larger halogens, especially as changes are observed for more elaborate platinum complexes, such as [PtX₃(C₂H₄)]⁻ where X is Cl, Br, or I, with larger halide ligands.^8,9 In addition to the increased size of iodine, which may induce steric strain into the molecular framework, iodine has a larger spin–orbit splitting and a higher polarizability, which may change the bonding and electronic structure of the platinum iodides compared to other platinum halides. While building an understanding of the evolution of the electronic structure of the platinum halides is of fundamental interest, platinum halides have also found applications in catalytic processes. For example, perovskites with Pt–I₃ active sites are highly efficient photocatalysts for H₂ production.¹⁰ Here, we study the electronic structure and photochemistry of the simplest platinum iodides, PtI⁻ and PtI₂⁻. We employ photoelectron imaging, which probes the electronic structure directly and, through its photoelectron angular distributions (PADs), also offers insight into the molecular orbitals (MOs) involved. Performing photoelectron imaging over a range of photon energies can often provide more insight into the electron loss dynamics, as we and others have shown in several cases, and we use a similar approach here.^11–17 

The photoelectron angular distributions (PADs) are particularly interesting in the context of metal complexes, where, in principle, the sensitivity to the electronic structure can provide insight into the chemical bonding involved.^18–20 Jarrold and co-workers have recorded photoelectron spectra for some transition metal and lanthanide clusters, including NiO⁻ and Gd₂O⁻, at different laser polarizations relative to the detector, in order to gain some information about the PADs and characterize the spectroscopic transitions.^21,22 Here, we record the full PADs, using photoelectron imaging, in order to investigate the symmetry of the molecular orbitals of transition metal complexes. Moreover, for the case of platinum iodides, spin–orbit coupling and relativistic effects are likely to be large and these can have striking influences on the molecular orbitals of such complexes, which again the PADs may be sensitive to.^23,24

Previous studies have considered the electronic structure of platinum fluorides and chlorides, PtF_n⁻ and PtCl_n⁻ (n = 1–8), using theoretical methods and photoelectron spectroscopy.^5,6 Strong similarities were noted between the platinum fluorides and chlorides. Both platinum dihalides exhibited linear anion and neutral ground states. The EA for PtF₂ was calculated to be between 2.72 and 3.13 eV,⁶ which is slightly lower than the experimentally measured EA ∼ 3.5 eV for PtCl₂.⁵ The previous photoelectron spectroscopic study of PtCl_n⁻ (n = 2, 4, and 5) used a single photon energy hν = 4.66 eV; the PtCl₂⁻ spectrum comprised of three distinct bands.⁵ Note that the n = 1 diatomic molecule was not observed in this previous work and, to the best of our knowledge, there have been no photoelectron spectra of any platinum halide reported. In general, stable MX molecules are rare and typically require a d¹⁰ electron configuration and bulky ligands in order to stabilize the cluster.²⁵ Here, photodissociation of PtI₂⁻ results in PtI⁻ and has allowed us to measure the photoelectron spectrum of a diatomic platinum halide for the first time.

Our work utilizes 2D photoelectron imaging, and the experimental apparatus has been described in detail elsewhere.^26,27 Briefly the anions were produced via electrospray ionization of a solution of 2 mMol K₂PtI₆ in methanol. Anions were desolvated in a capillary, transferred through a series of differentially pumped regions by means of ring-electrode guides, and stored in a ring electrode trap, before being accelerated and mass-selected in a Wiley–Mclaren time-of-flight spectrometer. The ring-electrode guides also serve to perform collision-induced dissociation.^26,27 In the present experiments, PtI₆²⁻ serves as a precursor to form other platinum iodide species, with PtI₂⁻ being one of the most abundant in the mass-spectrum. As described below, PtI⁻ was formed via the photodissociation of PtI₂⁻. Surprisingly, we did not observe PtI₆²⁻, suggesting that it may be quite unstable as an isolated dianion.

Photoelectrons were generated through the intersection of the mass-selected anion packet and a nanosecond laser pulse. Pulses of variable photon energies in the visible and UV were produced via a Nd:YAG pumped optical parametric oscillator (OPO). The photoelectrons were imaged on a dual microchannel plate detector in a velocity map imaging configuration,²⁸ and photoelectron spectra were subsequently obtained from these raw images. Photoelectron imaging also yields the PADs of the emitted electron relative to the polarization axis, which was parallel to the detector face. Previous studies of platinum halides used a magnetic bottle electron detector, which does not record the PAD and has a very low detection efficiency for photoelectrons with low electron kinetic energy (eKE).^5,21,29 The energy resolution of our photoelectron spectrometer is 5% of the eKE, based on calibration with the photodetachment of iodide.

The raw photoelectron images were deconvoluted with a polar onion-peeling algorithm,³⁰ which reconstructs the 3D Newton sphere of electrons from the 2D image obtained and consequently recovers both the photoelectron spectra and PADs. The PADs are dictated by the molecular orbital from which the electron is removed in the photodetachment process. The emission yield, I, as a function of angle, θ, between the outgoing electron vector and the polarization vector of the light field, ε, can be expressed as I(θ)∝[1 + β₂P₂(cos θ)], where P₂(cos θ) is the second order Legendre polynomial and β₂ is a so-called anisotropy parameter.^18,19 This latter parameter defines the PAD and is restricted between the values −1 ≤ β₂ ≤ 2. When β₂ = −1, the emission is predominantly perpendicular to ε(I(θ)∝ sin² θ) and when β₂ = 2, it is predominantly parallel to ε(I(θ)∝ cos² θ). When β₂ = 0, the emission is isotropic.

Photoelectron spectra of PtI₂⁻ were recorded with nanosecond (ns) laser pulses at photon energies between hν = 3.2 and 4.5 eV. Figure 1 shows two representative photoelectron images of PtI₂⁻ recorded at hν = 3.6 and 4.2 eV, from which subsequent photoelectron spectra have been extracted. All the photoelectron spectra are shown in Fig. 2 and reported as a function of electron binding energy (eBE), where eBE = hν − eKE. The spectra are normalized to the most intense feature in each spectrum, averaged using a five-point moving mean and are offset to allow comparison. There are several spectral features that are clearly visible over different ranges of photon energies. These can be broadly classed into three distinct groups: we have color-coded these in Fig. 2 and labeled A, B, and C, in the order of decreasing eBE. We now consider these in turn.

For hv > 4.1 eV, a cluster of three distinct peaks can be seen with maxima at eBE ∼3.5, 3.8, and 4.0 eV. The three peaks can be assigned to direct detachment: PtI₂⁻ + hv → PtI₂(E_int) + e⁻, where E_int accounts for the fact that the PtI₂ can be produced with varying degrees of internal energy. Within our spectral resolution, this internal energy can take the form of vibrational or electronic energy. The distribution of the peaks does not follow a clear Franck–Condon progression, and the peak spacing is much larger than may be expected for the low frequency vibrations typical of heavy triatomic molecules (cf. 14 meV for the symmetric stretch of I₃).³¹ Moreover, the three direct detachment bands have different measured β₂ values. From the hv = 4.4 eV photoelectron image, we determine β₂ to be −0.9, +0.3, and +0.4. Such dramatic changes are inconsistent with a vibrational progression. Hence, we conclude that the distinct peaks correspond to the formation of the neutral PtI₂ in different electronic states, where the differing β₂ values indicate detachment involving different molecular orbitals. As the photon energy decreases below 4.1 eV, the higher eBE features are no longer accessible. The hv = 4.1 eV spectrum, in particular, clearly shows the effect of the reduced detachment cross section at low eKE. Additionally, as threshold is approached the features have β₂ ∼ 0. Both these observations are expected from the Wigner law.³² The maximum of the peak with the lowest binding energy of the three peaks offers the vertical detachment energy, VDE = 3.5 ± 0.1 eV, and its onset offers the adiabatic detachment energy (electron affinity), ADE = 3.4 ± 0.1 eV.

For photon energies less than ∼4.0 eV, a new feature emerges peaking at eBE = 3.06 eV. This feature grows in relative intensity as the photon energy decreases, but this is likely just a reflection of the decreasing cross section of the direct detachment features (A) discussed above. The feature becomes much less prominent for hv > 3.9 eV and remains visible down to 3.2 eV. The spectral shape, binding energy, and β₂ parameters of peak B are consistent with the well-known photoelectron spectrum of I⁻.³³ This detachment peak is presumably formed via a multiple-photon process involving the two steps of photodissociation and subsequent photodetachment (here, we will use the term two-photon to describe such a process),

The presence of two-photon photodissociation and photodetachment of the resulting I⁻, below the ADE of PtI₂⁻, requires the presence of at least one bound electronic state of the anion, which is excited by the first photon. Dissociation may occur on the excited state potential energy surface or on the ground state surface following internal conversion; we cannot obtain information about the mechanism from our current experiments. As hν increases, direct detachment also becomes possible so that the anion excited states are now in the detachment continuum (i.e., resonances) and dissociation will compete with autodetachment.^17,34–37 The relative decrease of the intensity of the I⁻ feature compared to the direct detachment channels of PtI₂⁻ can arise from a number of reasons in addition to the one noted above: the favorability of a one-photon process over two-photon process, the large apparent photodetachment cross section of PtI₂⁻, and the absorption profile to the excited states of PtI₂⁻. At the low-energy spectral end, we did not succeed in acquiring a spectrum at hv = 3.1 eV, in part because our OPO has very weak output here and the cross section for detachment from iodide is low (near-threshold). Below hv = 3.06 eV, iodide would no longer be visible in our experiment. Hence, we cannot comment on the spectral range of the excited state absorption of PtI₂⁻, except to say that it spans at least 3.2 ≤ hv ≤ 4.0 eV.

Finally, we also noted the presence of a very weak signal in several of the photoelectron images. Figure 1 highlights this at hv = 3.6 eV, where a series of sharp rings are seen at large radii. In Fig. 2, these features are not immediately obvious because of their low intensity, but we highlight it in the hv = 3.5 eV photoelectron spectrum by scaling the signal in the relevant range by an order of magnitude. The low intensity of this feature suggests that it arises from another two-photon photodissociation and subsequent photodetachment process (similar to feature B). These peaks are present over the spectral range 3.4 ≤ hv ≤ 3.9 eV, but with the largest intensity at hv = 3.5 and 3.6 eV. The overall structure, at hv = 3.5 and 3.6 eV, consists of three distinct peaks with eBE = 2.35–2.80 eV. Each peak has a different anisotropy, i.e., β₂ = −0.8, +2.0, and +1.5 in the order of increasing eBE and extracted from the hv = 3.5 eV spectrum. However, we do note that the signal level for these features is very low such that the spectral structure and the PADs should be taken as qualitative rather than quantitative measurements.

As only Pt and I are present in PtI₂⁻, we first considered the possibility that the photodissociation product corresponds to Pt⁻, which is subsequently detached. However, the electron affinity of Pt has been measured as 2.12 eV, and the previously reported photoelectron spectrum shows different structure and relative intensity,^38,39 so the spectrum is not consistent with this explanation. The photoelectron spectrum measured is also not consistent with that of I₂⁻, which has been extensively studied.⁴⁰ This leaves us with the only other alternative, PtI⁻. To the best of our knowledge, this diatomic has not been characterized, either as an anion or neutral molecule. From our spectra, we determine that the ADE = 2.35 ± 0.10 and the VDE = 2.45 ± 0.1 eV.

Analogous to feature B, the most likely explanation for this feature is the two-photon process,

Hence, the excited anionic states accessed in PtI₂⁻ appear to lead to a competition between dissociation leaving the negative charge on the PtI or I, with the latter apparently dominating (although this is difficult to verify without knowledge of relative photodetachment cross sections of the anions). It may also be possible that PtI⁻ undergoes dissociation (either spontaneous or by absorption of a further photon) to form I⁻, and this would be indistinguishable spectroscopically from the two-photon process of photodissociation of PtI₂⁻ to form I⁻, although a photodissociation would be unlikely by the small probability of a three-photon process required.

The three peaks associated with the photodetachment from PtI⁻ with differing β₂ values are similar to the photodetachment from PtI₂⁻ (feature A). The spectral structure of the direct detachment channels for both PtI⁻ and PtI₂⁻ exhibits three peaks, with a β₂ < 0, >0, and >0, and a total width of ∼1 eV. This is perhaps unexpected as the electron configurations of the two molecules are different: PtI⁻ is an even-electron species, whereas PtI₂⁻ is an odd-electron species.

The direct photoelectron spectrum of PtI⁻ shares many characteristics of that seen for PtI₂⁻. Our measured ADE (and VDE) are also similar to those calculated values for PtCl⁻ (VDE = 2.21–2.70 eV, depending on level of theory),⁵ the only other diatomic platinum halide studied to date. As we were not able to form an ion beam of PtI⁻ directly via electrospray ionization, it is difficult to determine whether it undergoes photodissociation. However, this may be unlikely as ionic photodissociation could result in a neutral Pt atom, which is not a favored oxidation state of platinum.

Previous photoelectron spectroscopy of PtCl₂⁻ determined its ADE = 3.5 eV and its VDE = 3.83 eV,⁵ while computational work predicted the electron affinity of PtF₂ to be between 2.72 and 3.13 eV depending on the level of theory used.⁶ Our measured VDE = 3.5 eV for PtI₂⁻ is therefore of similar magnitude to these lighter halides. Clear similarities are also observed between the photoelectron spectra of PtI₂⁻ and PtCl₂⁻.⁵ The hν = 4.66 eV photoelectron spectrum of PtCl₂⁻ exhibits three distinct peaks due to direct detachment. The highest eBE peak was near threshold, and its low relative intensity may be skewed by threshold effects, as we noted in the hv = 4.1 eV spectrum in Fig. 2 for example. The two other peaks at lower eBE are well-resolved, and the overall width of the observed photoelectron signal spanning the three peaks is ∼1 eV. This overall appearance is similar to the photoelectron spectra observed for PtI₂⁻ (e.g., see Fig. 2, spectrum at hν > 4.1 eV, which has three sharp features with a total spectral width of ∼1 eV). The ∼1 eV total spectral width of the direct detachment bands for the dichloride and diiodide platinum complexes indicates that the electronic states in the neutral are not just spin–orbit split states arising from the spin–orbit coupling involving the halide.

To assign the three lowest lying electronic states of the neutral, we will focus on a simple molecular orbital (MO) picture for typical d-block compounds and refrain from performing ab initio calculations because of the large spin–orbit coupling and relativistic effects that are poorly accounted for in standard electronic structure calculations. In the d-block model, the nd valence orbitals of a transition metal, as well as any (n + 1)p and (n + 1)s orbitals with the correct symmetry to mix, are considered along with a symmetric ligand field based on an undercoordinated octahedral geometry.²⁵ From this model, a picture of the non-bonding and anti-bonding orbitals, which are typically the highest energy occupied orbitals and strongly localized on the metals can be developed, where the energies of the d-orbitals are determined by overlap with the ligand field. With reference to Fig. 3(a), the d_x²−y² and d_xy on Pt cannot interact with the atomic orbitals on I and forms a pair of degenerate non-bonding (n) MOs. The overlap between the d_xz and p_x as well as the d_yz and p_y orbitals will form doubly degenerate π and π* MOs. In the absence of orbital mixing, the overlap of the d_z² and ligand localized p_z orbitals may be expected to lead to σ and σ* MOs. However, the 6s orbital of Pt is close in energy to the 5d orbitals, as evidenced by the 5d⁹ 6s¹ ground state electron configuration of Pt, and of the correct symmetry to mix with the 5d_z² orbital, leading to a hybrid s/d orbital. This hybrid orbital in turn interacts with the p_z orbital on I to lead to three MOs: one bonding, one anti-bonding, and one non-bonding, as shown in Fig. 3(a) in red. These MOs, while being hybrid, effectively appear as σ-type MOs.

Taking the above arguments and applying it to the case of a linear ML₂ molecule with a σ bond and the potential for π interactions, as is the case for the d⁹ PtI₂⁻ complex, the d-block is split into four (n, n, σ*, and π*). From a simple Koopmans’ theory picture, electron loss from any of these orbitals in PtI₂⁻ could result in the formation of distinct neutral states and therefore a band in the photoelectron spectra. As three bands are observed in the photoelectron spectra (Fig. 2), the three highest energy MOs of the d-block of PtI₂⁻, which are likely to contribute to the photodetachment dynamics, are shown in Fig. 3(b). Both the anti-bonding and non-bonding s/d hybrid orbitals are filled, and one additional electron resides in the π* orbital (i.e., PtI₂⁻ is an open-shell radical). However, photodetachment from the lowest energy n MOs in the d-block (based on the d_x²−y² and d_xy platinum orbitals) is not observed in the photoelectron spectra, suggesting that there is insufficient energy to remove an electron from this MO. This may be surprising given the similar energy of the Pt 6s and 5d, but the d-block model is likely to be an oversimplified picture of the MOs, meaning that the energy difference between the n orbitals, one of which is a hybridized MO, is likely to be larger than implied in the simple model. Alternatively, the n orbitals are near-degenerate and both contribute to the highest eBE peak.

We additionally have measured the PADs for the three individual direct detachment peaks. Detailed qualitative and semi-quantitative models have been developed for the prediction of the β₂ parameter.^41–43 Given the qualitative picture presented above, we continue along such lines and consider the qualitative PADs that might be expected for the three different detachment channels. Qualitatively, β₂ parameters are expected to be positive or negative depending on the nature of the orbital from which the electron is detached. Generally, for an σ orbital, the outgoing wave can be approximated as a p-wave resulting in β₂ > 0; in contrast, for a π orbital, the outgoing wave will have a mixture of s- and d-waves, which results in β₂ < 0. For the three channels observed in the experiment, we observe β₂ = −0.9 for removal from the π* MO, β₂ = +0.3 eV for the removal from the σ* hybrid MO, and β₂ = +0.4 eV for the removal from the n MO. These observations are in qualitative agreement with the expectation assuming that the highest eBE peak arises from the n-hybrid MO. The PADs therefore offer additional support that the proposed assignment and the simple d-block picture are representative of the electronic structure of PtI₂⁻. It should be noted that the origin of the structure of the PtCl₂⁻ photoelectron spectrum was not discussed by Joseph et al.⁵ 

In this simple d-block model we have not accounted for the role of spin multiplicities, which would allow the observed spectral structure and PADs for PtI₂⁻ to be explained via photodetachment from just two MOs, instead of the three suggested by the above model. Specifically, the three bands could arise due to the removal of an electron from the π*, which would produce a ¹Σ neutral state, or from the σ*, which would produce ¹Π and ³Π neutral states. The observed PADs would also be consistent with this model, as the two bands originating from the electron removal from the same σ* would both have β₂ > 0.

However, the d-block model shown in Fig. 3(b), with or without the inclusion of spin multiplicities, does not consider the role of spin–orbit coupling, which may have a large effect on the nature and energies of the valence MOs of PtI₂⁻. For example, consider the addition of Russell–Saunders (LS) spin–orbit coupling to the HOMO and HOMO − 1 of the simple d-block and spin multiplicity model. In this picture, the removal of an electron from the π* or σ* d-block orbitals could result in ¹Σ₀, ¹Π₁, ³Π₀, ³Π₁, or ³Π₂ neutral states. In light of the large spin–orbit coupling observed in atomic iodine [E(²P_3/2) − E(²P_1/2) = 0.94 eV]⁴⁴ and platinum [E(³D₁) − E(³D₃) = 1.25 eV],⁴⁵ it may be expected that the different spin–orbit states of the Π symmetry will have significantly different energies but, as the Π states originate from electron ejection from the same σ* orbital, the PADs may be expected to be similar (as in the spin multiplicities model described above). Therefore, this combined d-block model with spin–orbit coupling could also explain the origin of the three observed bands and the measured PADs for PtI₂⁻. However, from this picture we may expect more bands in the photoelectron spectra of PtI₂⁻ than are observed (overlapping bands may not have been resolved). In addition, it is more difficult to explain the strong similarities between the photoelectron spectra of PtCl₂⁻ and PtI₂⁻, using this picture, as the Cl and I have significantly different spin–orbit splittings.

On a more fundamental note, it is questionable how valid the use of LS coupling is for PtI₂⁻, when heavy atoms such as Pt are best described by J–J coupling and the d-block is localized on the metal core. It should be noted that J–J coupling would also result in four spin–orbit split neutral states following electron removal from the σ*. One further consideration is the effect of spin–orbit coupling on the PAD, particularly in the limit of J–J coupling, where the orbital angular momentum quantum number (L) is no longer a good quantum number. PADs are often qualitatively interpreted in terms of the L of the orbital from which the electron is lost, and therefore, if J rather than L is well defined, it may be challenging to predict the PADs associated with specific photodetachment channels.

A very similar three-state d-block picture can be constructed for PtI⁻ [Fig. 3(b)], the highest MOs being π*, followed by a σ* hybrid MO and the n hybrid MO. Formally, the d-block would also contain a lower energy n MO, but this is not observed here, either because it is too low in energy or because the peak in the photoelectron spectra is obscured by the much higher intensity I⁻ feature (B in Fig. 2). The hybridization in PtI⁻ is slightly more complicated because there is likely to also be mixing with the 6p orbitals of the Pt atom. However, overall, given the similarity between the photoelectron spectroscopies of PtI₂⁻ and PtI⁻, our proposed picture appears appropriate. In Fig. 3(b), the MO occupancy is also shown, along with the likely photodetachment channels that contribute to the photoelectron spectrum. Analogous to the PtI₂⁻ detachment, the three peaks come about from the detachment of the π*, σ* hybrid, and n hybrid MOs. The PADs are expected to be similar again: β₂ < 0, β₂ > 0, and β₂ > 0, respectively. This is again in excellent qualitative agreement with experiment. It should be noted that addition of spin multiplicities and spin–orbit coupling to this three-state d-block model does not readily describe the observed spectral structure of PtI⁻, as LS coupling would predict two bands at the highest eBE with β₂ < 0.

Note that we specifically refrained from performing electronic structure calculations. This was done because of the lack of confidence we have in these, particularly for predicting the MOs that may be contributing to the detachment. Specifically, spin–orbit coupling, which in reality is likely to be intermediate in character between the limits of LS and J–J coupling, can lead to strong mixing of angular momenta. However, despite the likely large role spin–orbit effects have on the electronic structure of PtI₂⁻ and PtI⁻, it is very challenging to accurately account for these effects in electronic structure calculations. Therefore, instead of attempting to provide interpretations based on electronic structure calculations in which we have little confidence, we prefer the simple d-block picture, which we feel offers much more chemical insight too.

In addition to the direct photoelectron spectra of PtI₂⁻, we also observe secondary (two-photon) detachment from I⁻ or PtI⁻, indicating that bound (with respect to electron loss) electronically excited states for PtI₂⁻ exist (see Fig. 2, features B and C) and lead to two possible dissociation channels: I + PtI⁻ or I⁻ + PtI. From our experiments, we cannot gain any insight into the dynamics that leads to dissociation, and we cannot determine whether the dissociation occurs on the ground or the excited state. However, assuming that photodetachment from I⁻ and PtI⁻ has similar overall cross sections, the photoelectron spectra in Fig. 2 suggest that the I⁻ + PtI channel dominates. This observation is consistent with the fact that the electron affinity of iodine (3.059 eV) is larger than that of platinum iodide (2.35 eV).

A photoelectron imaging study of PtI₂⁻ is presented in the photon energy range 3.2 ≤ hv ≤ 4.5 eV. The VDE of PtI₂⁻ is measured to be 3.5 ± 0.1 eV, and the electron affinity of PtI₂ is 3.4 ± 0.1 eV. Three peaks contribute to the direct photoelectron spectrum, each with a distinct photoelectron angular distribution (PAD), which we can assign to the direct detachment from the anion to the three lowest lying electronic states of the neutral. Using a d-block molecular orbital model in which the 5d and the 6s orbitals on Pt interact with the 6p orbitals on I, a molecular orbital picture is constructed that is consistent with the observed spectrum including the PADs. The use of PADs to assign and understand the electronic structure of transition metal complexes holds significant potential in building up comprehensive pictures of such complexes.

PtI₂⁻ is also observed to undergo photodissociation to produce I⁻ predominantly, as evidenced by the photodetachment from iodide. This feature is visible over a large range of photon energies, including below the detachment threshold, indicating that PtI₂⁻ has at least one bound excited state with respect to electron loss. In addition to the loss of I⁻, additional features are seen over a spectral range near hv ∼ 3.5 eV. These have been assigned to PtI⁻, and a d-block model analogous to that for PtI₂⁻ allows us to assign the features and their PADs. The electron affinity of PtI is found to be 2.35 ± 0.10 eV.

The authors would like to thank Dr. Basile Curchod for valuable discussions about the electronic structure of PtI₂⁻ and PtI⁻. Jemma A. Gibbard is grateful for support from a Ramsay Memorial Fellowship.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.