On a physical theory of stellar spectra

Saha, M.

doi:10.1007/bf02709304

Cited by 19 publications

(23 citation statements)

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“…Under thermodynamic equilibrium, there is a coexistence of free charges and excitons, and an interchange between the two species. The fraction of free charges (n FC ) vs. excitons (n exc ) is not only determined by the binding energy, which determines how long electron-hole pairs remain bound, but also by the excitation density (n), which influences how often electrons and holes meet to form excitons 13,14 .…”

Section: Resultsmentioning

confidence: 99%

“…Consequently, we can safely assume that the binding energy is independent of excitation density 23,25 . Within such an approximation, we can apply the law of mass action for Wannier-Mott excitons, as described by the Saha-Langmuir equation 13,14 . The fraction of free charges over the total density of excitation, x, for a 3D semiconductor can be expressed as 13 :…”

Section: Resultsmentioning

confidence: 99%

“…Subsequently, we use this value in the Saha equation, which governs ionization processes 13,14 , to estimate the equilibrium branching ratio between free charges and bound excitons as a function of temperature and charge density. We find that at equilibrium, following photoexcitation, there is a predominant fraction, near unity, of free charges in the PV operating regime.…”

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confidence: 99%

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Excitons versus free charges in organo-lead tri-halide perovskites

et al. 2014

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Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic-inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50 meV. We show that such a value is consistent with almost full ionization of the exciton population under photovoltaic cell operating conditions. However, increasing the total photoexcitation density, excitonic species become dominant, widening the perspective of this material for a host of optoelectronic applications.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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confidence: 99%

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Excitons versus free charges in organo-lead tri-halide perovskites

et al. 2014

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“…In a clinical setting the laser is set to deliver in the high intensity regions fluence larger than H T in order to have consistent generation of LIOBs and corresponding clinical effect. The average expected value of seed electron generation in the epidermis will be calculated using the mathematical formalism for thermal ionization describing the density of electrons and ions of various level of ionization developed originally by Saha [12,13] and later expanded for microscopic solid particles [14]. The Saha equation for the number density of free electrons n e , singly ionized melanin n i , and non-ionized melanin n can be written as…”

Section: Resultsmentioning

confidence: 99%

A comparative study with a 755 nm picosecond Alexandrite laser with a diffractive lens array and a 532 nm/1064 nm Nd:YAG with a holographic optic

Tanghetti

Jennings

2017

Lasers Surg Med

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Background and Objectives: This study was performed to better understand the cutaneous effects of using a fractional picosecond laser at 755 nm with a diffractive lens array and a picosecond Nd:YAG laser at 532 mn and 1,064 nm with a holographic optic. We characterized the injuries created by these devices on skin clinically and histologically over 24 hours. With this information we modeled the effects of these devices on a cutaneous target. Methods: Eight patients, representing Fitzpatrick skin types I-VI, were treated on their backs with a picosecond Alexandrite laser with a diffractive lens array, as well as a picosecond Nd:YAG laser at 532 nm and 1,064 nm with a holographic optic. Photographs were taken 15 minutes and 24 hours after treatments. Punch biopsies were obtained at 24 hours and examined histologically. Results: Treatment with the picosecond Nd:YAG laser at both 532 nm and 1,064 nm with the holographic optic revealed erythema and small scatted areas of petechial hemorrhage areas immediately and in many cases at 24 hours after treatment. The 755 nm picosecond Alexandrite laser with diffractive lens array produced erythema immediately after treatment, which largely dissipated 24 hours later. Histologies revealed intra-epidermal vacuoles with all three wavelengths. Fractional picosecond Nd:YAG laser at 532 nm and 1064 nm with the holographic optic showed focal areas of dermal and intra-epidermal hemorrhage with areas of vascular damage in some patients. Conclusions: This study demonstrates that both fractional picosecond devices produce vacuoles in the skin, which are most likely due to areas of laser induced optical breakdown (LIOB). In the patients (skin type II-IV) we observed scatter areas of hemorrhage in the skin, due to vascular damage with the 532 nm and 1,064 nm, but not with 755 nm wavelengths. Lasers Surg. Med. 50:37-44, 2018. © 2017 Wiley Periodicals, Inc.Key words: laser induced optical breakdown (LIOB); superficial hemorrhage; fractional picosecond laser; holographic optic; diffractive lens array INTRODUCTIONThe use of picosecond lasers with fractional optics have provided an opportunity to deliver high energy short pulses of light to the skin. The picosecond Alexandrite 755 nm laser with a diffractive lens array has been used successfully to treat acne scars [1], photo-damaged skin [2], and melasma [3]. We have described the creation of intraepidermal vacuoles, which appear to be the result of areas of LIOBs from the absorption of high energy 755 nm laser light by melanin in the granular layer of the epidermis [4]. This localized damage has been associated with the production of new collagen and elastic tissue. It is possible that the production of epidermally generated growth factors, cytokines, and chemokines could be responsible for these changes [1,5]. The immediate clinical effects of transient erythema, heat, and swelling with this device lasting less than 24 hours has been well characterized [5]. In individuals with skin types I and II with melanin index (MI) of 12 or less we ha...

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“…Numerical simulations [9] show that during the laser irradiation in a steady-state case the temperatures may easily reach a temperature region between 3000 and 4000 K. Using the Saha equation [10] the density of thermally excited (free) oxygen atoms and ions can be calculated. Because the calculation of the internal partition functions necessary to solve the Saha equation for the dissociation process of glass SiO 2 → Si + 2 O is quite involved, we will restrict our considerations only to the energetically most demanding process, namely the ionization of (already free) oxygen atoms: That this process is indeed energetically most demanding is shown in the following.…”

Section: Observations Of Underlying Processesmentioning

confidence: 99%

Gas bubble formation in fused silica generated by ultra-short laser pulses

Cvecek¹,

Miyamoto²,

Schmidt³

2014

Opt. Express

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During processing of glass using ultra-fast lasers the formation of bubble-like structures can be observed in several glass types such as fused silica. Their formation can be exploited to generate periodic gratings in glasses but for other glass processing techniques such as waveguidewriting or glass welding by ultra-fast lasers the bubble formation proves often detrimental. In this work we present experiments and their results in order to gain understanding of the origins and on the underlying formation and transportation mechanisms of the gas bubbles.

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On a physical theory of stellar spectra

Cited by 19 publications

References 0 publications

Excitons versus free charges in organo-lead tri-halide perovskites

Excitons versus free charges in organo-lead tri-halide perovskites

A comparative study with a 755 nm picosecond Alexandrite laser with a diffractive lens array and a 532 nm/1064 nm Nd:YAG with a holographic optic

Gas bubble formation in fused silica generated by ultra-short laser pulses

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