## 1 Introduction  
Due to their unconventional electronic structure, topological insulators (TI) are potential candidates for next-generation electronic and spintronic devices [1; 2; 3]. They behave like standard insulators or semiconductors in the bulk, but stimulating physics takes place at their surface. Here, the fermion gas forms a conductive state with a Dirac cone structure in the Brillouin zone and a helical spin texture. Recent studies have shown several unique features of TI, including observations of the quantum spin Hall effect [4], Majorana fermions [5], the topological magnetoelectric effect [6], magnetic monopoles [7], nonlinear modulation of the electromagnetic field [8], and the anomalous quantum Hall effect [9]. In practice, some difficulties have prevented the widespread use of TI in mainstream electronic components. The major obstacle is the large contribution of bulk electrons that hamper surface transport properties [10]. It has been shown that a reduction of bulk effects leads to better control of the interesting surface phenomena. In particular, focusing on the Bi\({}_{x}\)Sb\({}_{2-x}\)Te\({}_{y}\)Se\({}_{3-y}\) (BSTS) family, some of the most promising ternary and quaternary compounds were identified as Bi\({}_{2}\)Te\({}_{2}\)Se [11], Bi\({}_{x}\)Sb\({}_{x-2}\)Te\({}_{3}\)[12], Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{1.8}\)Se\({}_{1.2}\)[13] and BiSbTeSe\({}_{2}\)[14]. These investigations have shown that fine-tuning the BSTS stoichiometry allows one to control the Dirac cone structure and, consequently, the electronic and spin properties of the TI [15; 16; 17; 18].  
From the experimental point of view, angle-resolved photoemission spectroscopy (ARPES) has been a key tool to disclose the main features of TI, as it provides a direct picture of the occupied electronic states in the Brillouin zone [19; 20; 21]. Being conductive, the topological surface states (TSS) are only partially filled. However, complete knowledge of their structure would be highly desirable, especially if one wishes to tailor specific charge and spin transport properties. To overcome this limitation an advanced approach combines ARPES and pump-probe techniques, resulting in the so-called time-resolved ARPES (TARPES). Here, the sample is perturbed by a short laser pulse--the pump--which promotes electrons to unoccupied states above the Fermi level. Recording ARPES images during the excitation and relaxation processes, one can access the electronic structure of both occupied and empty states, and also reveal the main electronic scattering processes as thermodynamic equilibrium is restored after photoexcitation [22; 23; 24; 25; 26].  
In this paper, we investigate the ultrafast electronic dynamics of selected BSTS samples by means of TARPES with sub-100 femtosecond time resolution. The binding energy of the Dirac node (DN), i.e., the energy where upper and lower branches of the Dirac cone meet, can be tuned from \(-0.3\) eV to \(+0.2\) eV, depending on the BSTS stoichiometry. The electron density of states (DOS) is drastically reduced in the vicinity of the DN, leading to a strong alteration of electronic interactions [27]. Therefore, BSTS samples offer an unprecedented opportunity to investigate the electronic behavior of the TSS in relation to its filling, i.e., to the position of the DN with respect to the Fermi level. Real applications require long-lasting electronic dynamics at a TI's surface (which usually only last a few picoseconds). Our results show that Dirac carriers relax on an extraordinarily long timescale when the binding energy of the DN is vanishingly small. We believe this novel observation might provide critical information to guide the fabrication of innovative TI-based spintronic devices.