## 1 Introduction  
Laser ablation of a bulk graphite target in water leads to the synthesis of carbon-based nanomaterials (NMs) (C-NMs), such as carbon nanoparticles (NPs) (CNPs), graphene quantum dots (GQDs), graphene layers, and others; also, the process leads to hydrogen-terminated polyynes [1; 2; 3; 4]. Polyynes are molecular compounds consisting of chains of sp-hybridized carbon atoms with alternating single and triple bonds (a series of consecutive alkynes [(\(\cdot\)C\(\equiv\)C\(\cdot\))\({}_{n}\)] with \(n>1\)) that are end-capped by hydrogen atoms. Methyl [CH\({}_{3}\)]-terminated polyynes have also been observed to be formed but with a probability that is lower by almost two orders of magnitude compared with that of hydrogen-terminated polyynes [4; 5]. Polyynes exhibit a semiconducting behavior that depends on the end groups. The synthesis of C-NMs and polyynes by laser ablation of a bulk graphite target in water has been well-investigated in the case of using nanosecond (ns) or picosecond (ps) laser sources for ablation. These carbon nanostructures have excellent electronic, optical, and mechanical properties and a potential applicability in optoelectronics, biosensing, organic electronics, nanocomposites, and other fields [2; 4]. A straightforward and easy method to identify them is by using UV-vis absorption spectrophotometry measurements of the produced solutions; short chain polyynes produce a series of sharp peaks in the spectra in the region of \(\sim\)190-240 nm, whereas C-NMs produce wider bands in the region of \(\sim\)240-300 nm [2; 3; 4]. Early studies of the laser ablation of a bulk graphite target in water for polyyne and NM synthesis showed that, upon ns laser ablation (5, 6, or 7 ns; 1064, 532, 355, or 266 nm; 10 Hz; 0.4-5.1 J/cm\({}^{2}\); irradiation time of 5-60 min), short-chain polyynes (C\({}_{6}\)H\({}_{2}\), C\({}_{8}\)H\({}_{2}\), and C\({}_{10}\)H\({}_{2}\)) were synthesized [6; 7; 8]; meanwhile, the efficiency of the shorter chain polyyne formation was enhanced while ablating the target in an acidic solution compared with a basic solution due to the higher H\({}^{+}\) concentration that occurred as a result of the faster chain growth termination [9]. A background residual absorption in the UV-vis absorption spectra was attributed to the formation of carbon-based NPs in addition to the polyynes [6]. Subsequent studies showed that upon the ns laser ablation (5 ns/532 nm/10 Hz) of highly oriented pyrolytic graphite (HOPG) in water with fluences below \(\sim\)6 J/cm\({}^{2}\), only polyynes (C\({}_{8}\)H\({}_{2}\) and C\({}_{6}\)H\({}_{2}\)) were formed, whereas with fluences above \(\sim\)6 J/cm\({}^{2}\), C-NMs (reduced graphene oxide (rGO) layers) were also formed together with the polyynes [10]. A study of the laser ablation of HOPG in water using a fs laser source (35 fs/800 nm/1 kHz) had presented UV-vis absorption spectrophotometry spectra where, for a fluence of 25 J/cm\({}^{2}\), C-NMs in the form of GQDs and porous graphene (PG) nanosheets were formed in the solution [11]. More recently, a ns laser ablation (6 ns/532 nm/10 Hz) of graphite in water with a fluence of 0.4 J/cm\({}^{2}\) led to the synthesis of C-NMs (nanosheets, nanospheres, nanotubes, and nano-necklaces) (absorption peak at \(\sim\)266 nm) without the presence of any peaks from polyynes in the spectra [1]. Upon the ps laser ablation (700 ps/1064 nm) of a graphite target in water with fluences in the region of 0.41-0.95 J/cm\({}^{2}\), C-NMs in the form of CNPs (mean size of around 18 nm) were formed for a pulse repetition rate of 7 kHz, but no polyynes were formed; meanwhile, for higher rate of 10 kHz, polyynes were also formed together with the carbon nanostructures [12].